The sedimentary environment and evolution of the Manitou Passage area of Lake Michigan
French, William Edwin, 1936-, University of Michigan. Great Lakes Research Division.

Page  I THE2 SFDDJ2-NTARIY B7,11RO"22T A91) GEOLOGICAL EVOLUJTIOIT YAWT.TOU PASS~k2A-REkA 0? L.A:C M, iIG by o7ii Iiwn E 7ivin Fr-e nc h A dicssrtat'ion ~,tubrnitted in~ p a~rti a fuiI f limn ent of the recquiromient for th-"Iello ~e e o of Doctor of PhlAoso~phy i n the Un~ivernity of!M~ichigjran 1965 hotoral corcit teo: Profe:-zor Jacl: L. 1`011h, Ch ai rnman Profoc;;or John C. A ~o r.n,?rofon-,ior Loui3i X. TDri-go Do-ctor Charles~ F. Poworj

Page  II PREFACE This study is a part of the geology section of the coherent area study of Lake Michigan of the Great Lakes Research Division, Institute of Science and Technoloy, University of M.ichigan. The work was made possible by U. S. Public Health Service g-rant:VP-00311. The coherent area progran is under the direction of Professor John C. Ayers, Principal Investigator and Professor David C. Chandler, Project Director. The first year of this geological study was under the direction of Professor Ayers and the remainder under Professor Jack L. Hough. IDring the 1.963 season, diving assistance was contributed by Mr. Robert Anderson Jr. and Ir-. 'tallace P. Vells. *:r. Louis Warnes of Glen Haven kindly contributed storage for the small boat and diving equipment. l'r. Robert Anderson Sr. of Traverse City permitted the writer to mJake aerial observations and( photog raphs from his airplane. Field assiista:ce in 1964 was provided by the crew of the R/-J TTAIAD which included the wr:iter, e Sor, Paul trmmers, Michael Srnith, and John Osterhagen. The 1964 meter installation was made possible by the excellent cooperation of the U. S. Coast Duard, and especially the pers:onnel of the NTorth Manitou Shoal Lilght Station, George T. Gautier, Officer in Char;e. The 1963 current meter was constructed with the assistance of the Department of Geology, University of Illinois. ii

Page  III CO0NTENTS page Introduction 1 deological SettinC 7 Topography- 7 Sub-bo-tlt oml cecloz1 y 20 Sediment distribution 22 Currenta 27 Sleeping Bear Shoal 27 North N-.anitou Shoal. 29 Conclusionnc 41 TABLBS 1 Current Rerequoncy at Sleetping Bear Shoal, 8 i u j-4t through 4 September 1963 29 2 Wijnd frequency at "North Nlanitou Shoal, 5 I:ay through 5 Nove~mber 1964 33 3 Wind frequency at Ni'lorth lM'anitou Shoal, 26 July through 26 Septemnber 1964 35 4 Current frequ-,ency at No rthll M'anittou Shoal, 30 July through 26 September 1964 38 Ji. J,

Page  IV FI GURED page 1 Sleeping Bear Hill and Sleeping Bear Point 3 2 General Bathymetry of lanitou Passage 4 3 Ripple marks on Sleeping Bear Shoal 9 4 Irregular bottom on North aTranitou Shoal 11 5 Vicinity of Sleeping Bear Point 12' 6 NTear-bottom current studies 14 7 Ridges on north slope of Sleeping Bear Shoal 16 8 Fence diagrar of Sub-bottom horizons 21 9 Location of sediment samnp.es 23 10 Phi iedian distribution 24 11 Phi. Deviation distributiLon 26 12 Current rose for Sleeping Bear Shoal, 8 August through 4 September 1.93 30 13 Current meter in place on NTorth lianitou- Shoal 32 14 Wind rose for Ntorth TrManitou Shoal, 5 I.ay through 5 November 1964 34 15 Wind1 rose for North 'anitou Sh'oal, 26 July through 26 September 1964 36 16 Current frequency histo,.ramn from N orth L anitou Slioal, 30 July through 26 September 1964 37 17 Profile of the west face of Sleeping Beatr Hill 42 APPE?,DIX eolchanioal comlposition of thoe sediment samploo 45 iv

Page  1 INTRODUCTION In 1963 the Great Lakes Research Division began a broadly based study of Lake Michigan. The program includes meteorological, physical, chemical, biological, and geological investigations. The aim of these studies is to observe and interpret the processes in action at present in order to gain some insight into the history of the lake since its formation and its probable future evolution. The geological portion of this study involves two general types of operation. An extensive survey of the entire lake is being undertaken by means of sediment and bedrock bottom sampling, echo sounding, and continuous seismic profii ng of sub-bottom structure. In addition, selected portions of the lake are being subjected to intensive investigation to obtain a better understarding of the details of structure, sediment distribution, and sedimentary environm.ent. The work described here involves one such study made in the 1..anitou Pacs.:,ge area of the northeastern side of the lake. The southern half of the shoreline on the east side of the lake is in a comparatively advanced state of development, considering the short span of post-glacial time since the formation of Lake.ichigan. The northern part, however, is not as regular and displays the embayed nature of an immature shoreline. Investigations of the beach sands and offshore sediments of the southern part of the eastern shore are presently being carried out by the University of Illinois, Department of Goology, working in cooperation with the Creat Lakes Research Division. Theso studies are intended to yield information concernirng the processes at work lalong the more maturely developed parts of the shore. 3

Page  2 2 The present work was concerned with the M.anitou Passage area of the northern part of the east shore during 1963 and 1964. The ZYanitou Passage area is located off the northwestern shore of the Lower Peninsula of iichljan between the Leelanau Peninsula and the Manitou Islands. The position 86 W. long., 45 N. lat. is approximately the center of the area. This study, covers only the area lying between the mainland and the southern shores of the islands plus part of the shoal area west of Sleeping Bear Point (figs. 1 and 2). Detailed work was carried out chiefly. in the southern part of the passage within a few miles on either side of Sleeping Bear Point. The area was selected for this study because it is the point where the eastern shore of Lake ~:izhigan change:; from a predominantly northerly to a northeasterly trend.. This charge in direction is accompanied by transition from the relatively regular saind hill bordered shorelines to the south, to an irregular,.deeply indented shoreline to the north of the area. In addition, it wa::; nown from previous brief visits to the area that the currents aro active andt that the bottom topography warranted investigation.. urthermore, there are no large settle.::.nts, shoreline improvements, or drerdged. channels in the area. It is essentially in its natural. state, unmodified by man.. Insofar as it has been possible to determine, there have been no previous underwater investigations i n the area, except for U. S. Lake Survey soundings.. There is a map of the glacial geology of Leelanau County (Kelley, 1957), and inves:tigationn ha ve been made of the Glon L,-ke-SloSeping Boar Bay ernbaymont (JohnSon, 1958) and of Sleeping Boar Hill (Gillis and Btkeman, 1963).

Page  3 3 Fisue, ea Hllal(!Slepng3-.a Pon APyraiai3 Pont s J~ th riht bckl~,-rundant Pyr Maidtoit 1s3. nit, the right, bacl%-rcunand. Th e I1963 meter site i in the, lowier left carn 'r o~f the p2cture.

Page  4 4 MANITOU PASSAGE BATHYMETRY GONTOUR INTERVAL 50' MILES Figure 2 General Bathyrnmetx'y of Mnitou Passage The abbreviationo used are; NMS-North SYS-South Tanitou Shoal, SBS-Sleeping PPS-Pyrumnid Point Shoal, Manitou Shoal, Bear Shoal, and

Page  5 5 The Manitou Passage investigation was undertaken with the object of gaining somp idea of the magnitude of the forces and processes which determine the sedimentary environr.ent and the meteorological regime which powers it. Tnree major objectives were proposed. First, it would be necessary to ascertain the wind regime of the area. Since there are no nearby weather stations this was first attempted through use of the wind observations of the North Yanitou Shoal Light Station of the U. S. Coast li ard..owever, observations are taken only at four hour intervals and installation of a continuous wind recording system was seen to be a nece;slty. Tnhe second and third objects of the study were the measurement of the near bottom currents of the area and the relation of sediment distr iution to these currents. The major part of the sedimentary activity would be expected to take place when the most intense current and wave action occur, and these severe tinres are precisely those -when research vessels and divers are unable to operate. it was necessary, therefore, to locate a current meter on the bottom in r to orer o tain records affording continu — ity and measurements, of currernts under extreme conditions. No available current meters were suited to this type of installation, and it was.necesary to design and construct one to meet the requirements of this study. Since only one current meter could be constructed, it was impossible to measure the over all current pattern of the area. A further purpose of this investigation was to devise,, perfect, and employ direct observativn methods of underwater investigation. The use of SCU3A (self-contai.ne un derter breathing apparatu-) diving techniques in particular permits direct obse-rvation of osdimontary conditions on the bottom and truly repre:ontative nsmpling of the

Page  6 6 sediments. It also makes possible the observation of the small scale configuration of the bottom and the condition of objects lying on the bottom. The diver has somewhat the sLnme comprehension of the nature of the sample area as does the land geologist. H.owever, the diver is greatly limited by the fact that his visibility is very restricted and affords no over —all view of his area. There are also limitations on the number of dives which can be made in a day and the length of time which can be spent on the bottom during deep dives. Diring the sumrner of 1963 most of the work consisted of field investigation of the lake bottom using SCUBA gear. The writer with one -or more diving companions mad-e a number of dives in the Sleeping Bear 3ay and Sleeping Bear Shoal areas. Some of the dives in the bay were made directly from the shore. Otherwise a sma-ll boat equipped with an outboard motor was laulnched from shore in the bay and used to transport the divers to the diving site. Most of the information gained from these dives consisted of observations which were recorded in a waterproof notebook and,some underwater photographs. Tbr bottom sample collection, placement of the current rmeer and taking echCo sounding profiles, the R/V NAIAD made three short trips to the passage from her base in Grand Traverse Bay. The small boat was employed to recover the current meter on 25 October. For the 1964 season the AIAD ws assid to t anito Passage study on a nearly full time basis. Operations were based at Charlevoix under the direction of the writer. Cruioes. with a planned dur-tion of 4-5 days wore made to the passage. Unfortunately the distance from Charlevoix to the passage necessitated long runs to and from the working area which, combined with the unexpectedly severe weather collmonly found in the area, severely hampered oper.ltions. Anl a result complete

Page  7 7 coverage of the passage area was not possible. Diving, echo sounding, and sample collections for the 1964 season were performed from the NAIAD. The continuous seismic profiles were made from the R/V INlLAND SEAS which also took some bottom samples and recovered the current meter on 5 November. The data obtained during the two years of the investigation is on file at the Great Lakes Research Division. Selected portions of it are presented in this paper. A short paper based on some of the 1963 data was presented by the vriter (iFench, 1964). GEOLOG ICAL SETTING Topography The shoal areas showt rem:rkable uniformity of depth and bottom character. They are flat and their surfaces are essentially level throughout their extent. Surface relief is only a few feet over the entire shoal with no abrupt changes in level. Depths range from about 30 feet at the outer edf;e of the most exposed areas (Sleeping Bear Shoal and South anitou Shoal) to around 15 fe.t at the landward edes of Torth nlanitou Shoal and Pyramid Point Shoal. Except for the connecting sill between Sleepirn Bear Shoal. and South lLanitou Shoal, the margins of all the shoal areas are marked by abrupt dropoffs to depths of at least 100 feet. Where these slopes were examined by divers, they consisted of sand resting at, or close to, the angle of repose.

Page  8 8 The surface of each of the shoals rises gently towards land to a point a few hundred feet from shore, then slopes with a somewhat higher gradient up to the shoreline. In all cases, the shoals adjoin shorelines, which have bluffs over 100 feet high. Sleeping Bear Shoal which was the most intensively studied, appears to have two dominant bottom sediment types. The very center of the shoal is covered by a continuous pavement of cobbles, small boulders and pebbles. Diver examination shows that the pavement is essentially one cobble thick (about 20 cm). The material below this cobble is reddish brown sand (i.'unsell designation 10R 4/4 wet, 10R 6/4 dry) containing about 10 percent fines. The material is quite firm in place and was described in field notes as a firm sandy clay because the diver had to use a knife blade to dig out a sample (sample 032, appendix). Subsequent analysis, however, showed the material to be a fairly well-sorted medium sand containing a few granules and small pebbles, and only 10 percent fines. It should' be noted that this material is substantially identical to that which is obtained by digging into the face of the bluff on the west slope of Sleeping Bear Hill. The firmness of the material in place suggests that it is highly compacted and is the sandy.till which composed the original, post-glacial, topography of the area. Akvay from its center the remainder of the top of Sleeping Bear Shoal is covered by' very uniform, well-sorted, mediumn sand. Attempts to dig through this cover of sand were unsuccessful so it must be at least several feet thick. The surface of the sand i. covered by sharp-crested symmetrical ripple marko (fig. 3) about 20 cm apart. The crests of the ripple marks stand about 4 cm high. The marks are, in general, parallel to the

Page  9 9 IThigtxre 3 Rpl aL;o SloiC01nr B3ear Shoali lstn-tee between c rests S ot 20 rc

Page  10 10 crest-lines of the large surface water waves which were last active in the area. After a strong southwest wind which produced large waves, the crests of the ripple marks trend northwest-southeast, while after a northwest blow, the ripple marks trend northeast-southwest. Yithin a few hundred feet of the shoreline, the ripple marks always parallel the shore. Observations were also ma.de on the southern part of I.orth -t'anitou Shoal in the vicinity of Uorth i',anitou Shoal Light Station where the current recording apparatus was placed during 1964. Conditions here are more vigorous than those found on Sleeping Bear Shoal. The sand here is medium-coarse with frequent pebbles. As can be seen in figure 4 the surface of the sand is quite irregular with no strong development of ripple marks. Diver observations were imade in a depth of about 20 feet at the 1963 cuarrent meter site on Sleeping Bear Shoal (fig. 5). 'The sand here is medium-fine and conditions can be considered as fairly typical of the sandy parts of the sho-al in general. These observations consisted chiefly of measurement of the current flow immnediately above the bottom by observing the movement of s:-all puffs of dye. A very smnall amount of Rhodanmine-B dy-e was ejected from the end of a capillary tube which was positioned either just below the surface of the sand or.soreewhat above it. The miovement of the dye puff along a measured distance on the bottom was timed and the outline of the puff sketched at intervals after the original introduction. IDe which was injected just above the surface of the sand moved off down-current with an oscillatory motion. These oscillations had the same timing as the passage of the surface water waves that existed at the time. The length of each current oscillation was about the fname

Page  11 I Pi urol 4 lrroeimlr bottom., on:;.ortb llmmai.tuZo2 *UndrlIo-'wter te~clephoro c~able il coa r of Picture is about 3c-1 i n d i arie ter.

Page  12 12 Figure 5 Vicinity of Sleeping Bear Point Location of 1963 current meter inetallation is shown.

Page  13 13 as the distance between the crests of the ripple marks. Dye which was injected into the sand appeared at the surface intermittently and emerged with a similar oscillating movement, indicating that the interstitial water of the sediments vwas participating in the movement of the bottom water. Superimposed on this oscillatory motion was a general. drift which was always at a considerable angle to the direction of the oscillations. The net movement of the dye puff was, therefore, in the form of a flattened spiral the long axis of which was almost parallel to the crestlines of the ripple marks. Dye puffs ejected 50 cm above the bottom had a considerably greater arplitude of oscillation with a less flattened trajectory and moved off with the general drift of the water at a significantly greater velocity than puffs released nearer the bottom. rigJr e 6 shows the outline of a dye puff 30 seconds after it was (A) injected just below the surface of the can,- a t point 0; (3) injected 3 cm above the surface of the sand at point 0; and (C) injected 50 cm above the bottom at point 0. This 30 second period of observation effectively elintin-' ates the effect of the current oscillation and only showus the general drift of the water mass. Tihe lack of a t,.il on puff (C) clearl'y sh}ows that the cone of maximum shoear occurs within about 40 cm of the bottom. The fact that the inter-stitial water of the sediment ta:es part in the near-bottom oscillatory current would seem to indicato that the surface layer of sand on the bottom would be subject to a lifting motion during part of tho time and would more easily be placed in rsu;pension than might be as;unmeod from simple consideration of the shecr forces acting on an imperm.eable bottom.

Page  14 14 Current.- -m.. 0.5 mn I(A)' 0 1 2 m 0.5 m - l(B) — % c2::c~ Dy e --- -- -. - I 6 0 I 0 I 2 m (c) Bottom - D.y e -ta I I I 0 Figure 6 llear-bott-om ciirr,)nt studies

Page  15 15 These observations could be made only during relatively mile conditions. Winds were generally force 3 (12 mph) or less with a" - less than 0.5 m high. The surface current under these conditions was generally about 15 cm/sec and near bottom currents on the order of 5 cm/sec. Conditions this mild existed only about 40% of the time during the relatively calm month of August, with velocities five times as great existing 20%fo of the time. Conditions at this site during the much more windy time of fall and winter can only be surmised; wave motion must be on a much larger scale at such times. The edges of the shoal areas are terminated very abrupntl y,y a sharp increase in gra:dient. Within a horizontal distance of a few feet the level bottom changes to a slope of 30 - 40. This steep slope usually descends smoothly to a depth of somewhat over 100 feet.. From this depth a less regular slope extends to the floor of the Jajoining basin. It was possible to examine only the upper 100 feet of the slope by diving. This part of the bottom consists of mediutm-fine sand with up to.5% silt, laid down with a very smooth firm surface. This surface has a pattern of ridges. on it which trend directly down slope. Tear the top of the slope, these ridges are about 10 crn high and 50 cm crest-to-crest (fig. 7). Further down the s;lope in a depth of 70 feet they become larger w*wth crests about a meter high spaced several meters apart. It was not possible to observe whether or not the smaller ri.dges were continuous with the larger ones. The ridges( do not resemble ripple marks in that they were not observed to either branch or terlminate. Their profile in symrmetrical but the ridges.a.re not Sharp-cre.,ted. The ridges appear to have the same dgroee of roundness as thle trough, between the,,. iAother retLuon for doubting that theso ridcoge aro true

Page  16 16 Ii 4 I I` I.4:4 -1 I I.1 'I?iio7 io s o n n or th slo pe o f Sl3.eopin,g- >ce r Shoa

Page  17 17 ripple marks is that near the top of the slope the ridges trend nearly at right angles to the conventional sharp-crested ripple marlcs found on the edge of the shoal only a few feet away. Possibly these ridges are something similar to the sand waves which are observed to form in streams and channels which have very high current velocities. This would require occasional high velocity currents moving along the slope in an easterly or westerly direction. In a depth of about 100 feet near the base of the steepest part of the slope, hewn timbers were observed projecting from the bottom. Probings around these tirbers, which appear to be parts of wooden ships, showed that they are embedded several feet into the bottom. Assuming a maxin,'ur credible age for the timbers of less than 200 years, the sedimentation rate must be at least two feet per century. Compared with nearby submerged pilings of known age, it would appear that the wood is probably less than 50 years old, yielding a proportionally greater sedir.entation rate. It was not possible to observe the currents on the slopes as carefully as those on the shoals. It was seen, however, that the current which had come across the shoal did not follow the slope downward but, flo-,ed horizontally out above the slope. Indeed, under those conditions, a slight current was observed to run up the slope in a direction opposito to the main current coming off the shoal. This mig;ht represent an eddy current powered by the jet. effect of the main current where it separates fromn the 3a e bottor at the top of the -slope. These mrinor upslope currents wore generally loss than 5 cm/ooc. Sleeping Bear Bay and South YVanitou Island Harbor are the only places in the passage whore the land adjoins deep water without an

Page  18 18 intervening shoal. Ihe western part of Sleeping Bear Bay was examined in detail by divers (fig. 5). In the southern and vestern part of the bay there is a narrow band of shallow water paralleling the shore. Thlis extends to a depth of 15-20 feet and is'generally less than 100 feet wide. At the outer edge of this shallow area there is an abrupt increase in the slope of the bottomr similar to that at the edge of the shoal areas. This slope extends to a depth of 30-40 feet at essentially the angle of repose. Although the angle of the slope was not accurately measured, it is considered to be at the angle of repose because of the fact that any effort, by a diver, to dig into the face of the slope caused the entire area of the slope above the point of digging to slu:.p and slide downward. -The face of the slope is very nsmooth and free from vegetation. 'Iho sand of this slope is essentially identical to that of the slopes bordering the shoal areas, having up to 15% silt and having a smooth firm surface. From the base of this slope, the bottoten drops at a lesser gradient outward to the deep central part of the passage. At the base of the steep slope the bottoe is covered by a system of parallel ridges trending roughly northeast-southwrest. Vegetation which is identified.as algae of the famnly Characene grows on the present crests of the ridges. The ridgss are about 40 cm apart and rise 5 cm above the intervening troughs which are completely free of algae. The algae forms a dense mat of intertwined filaments which rise about 10 cm above the surface of the sand. Remain 'of these filamentse can be traced downward a considerable distance into the sand. It appears that this mat of filaments would be a very effective sediment

Page  19 19 trap since any sand which might settle into it would be very unlikely to be re-eroded.. Possibly the algae originally established itself in the hollows between the ridges (where an accumulation of organic material is found) and by causing preferential deposition brought about an inversion of topography to the present state. The ridges are observed to run under the base of the steep slope which is apparently encroaching on the area. Outward from a depth of' about 50 feet the vegetative cover becomes a continuous mat of filamentous algae and no marked ridge system is found. In this area, at a depth of about 45 feet, a partially buried anchor was found. Tle anchor was recovered and it is estimated that it had been covered by about 1.5 feet of sand since it was emplaced. Local inquiry established that this anchor is probably one which was used for shifting lumber ships prior to 7World War I. This would imply a present sedimentation rate of 2-3 feet per century. The deep central basin of sManitou Passage is the roughly triangular area bounded by the 250 foot depth contour, situated between Sleeping Bear 3ay and South.:anitou Island (fig. 2). It was not possible to examine the bottom at this depth by either diving or photography. The floor of the basin slopes gently from a general depth of about 2PO feet around its periphery to the center where the depth just exceeds 310 feet. Sediment samples t.aken in the southeastern part of the basin show t'at the bottom material is quite uniformly a si3t which contains about 25/o sand and less than 5%/ clay. j a in es, /

Page  20 20 Sub-Bottom Geologoy In the southern part of the area several traverses wore made with a Continuous Seismic Profiler or "Sparker." Figure 8 is an isometric fence diagram of these profiles. In plotting these profiles no correction was made for an increase of sound velocity in the sediments, because it is believed that the sonic velocity of lightly compacted sediments is not very much greater than that of the water. The profiles show the relative position and approximate depth of the sub-bottom horizons. The Sleeping Bear Shoal profile R-Y and most of Y-Z has essentially no sub-bottom detail. The tentative horizons s-hown may well represent spurious multiple echos of the bottom, and in any case are not very extensive. There is a strong sub-bottom horizon under the northern slope of the shoal, however, which is generally steeper than the present lake bottom and appears to be truncated by the top surface of the shoal. Tnis horizon appears to be continuous with one in the deep basin of 1'anitou Passage at a depth of about 100 foet below the present lae bottom. The continuity of this horizon suL,gests that it might be the original post-glacial surface 'of the passagbe area. Section L-K which pas.'e,? close to the tip of Sleeping Bear Point suggests that the point is essentially a pile of sand which is being built out over the pre-existirng l1ke bottom. The southcasterly parts of sections Ir-K and J-K show that within Sleeping Bcar Bay, there is a tendency to smooth the topography by truncating hills and filling hol lows. Under the essential3y level part of the floor of the deep basin (depths greater than?80 feet) there is a persistent horizon 20-40 foot below the present bottom. This pinches out around the edges of

Page  21 llplq. I.04 t J,( 51I - (I 150 JVVl 1D1ptl 1 feet 200 Figure 8 Fence diagram of Sub-bottom horizons

Page  22 22 the basin and evidently indicates some change or hiatus in the sedimentary filling of the basin. It may well represent the Chippewa low stage when the passage probably contained a lake separate from the main body of Lake Chippewa (Tough, 1955). Sediment:Distribution Bottom samples were taken at the points shown in figure 9. The samnples were taken either with an Orange Peel Dredge lowered from the AIAD or I:,n.C:D S-AS or by a diver scooping a sample with a trowel. In either case the sample is representative of the top few inches of the sediment. 5The sediment samples were analyzed for particle size distribution by a combination of sieving and hydrometer methods. Sand and granule sizes were sieved using the Tlyler Standard f2 series shaken by a RoTap Automatic.Shakin machine as described by Krumbein and Pettijohn (1933, p 138). Analysis for the silt and clay sizes was smade by the?7ouyoucos h ydrometer meethod as modified by the h;erican Society for Testing lraterials (1950). If pebbles were fould in a sample their presence was noted but no attempt at quantitative description of the pebble sziss was made. A cumulative curve wa3 plotted and statistical mearsures were calculated for each sample using the phi notation of Inman (1952). A tabulation of these meoasures is given in the appendix. Figure 10 is an i;oploth plot of the phi median particle cliameters of the sediments. The shaded areas represent bottom material com;posed of cobbles and boulder and aare considered to be areas of non-deposition. 1Te sandls of the area are componod dominantly of qua-rtz, with

Page  23 23. L&igh.160 150 151 152.153.1514 103 *155 *157 J.02 J0.156 U 0 0 -.11.0.9... a C 143 105/ 032 ' 000 a 002 0 1 c~3. 0 003I Fi gur e 9 Location-of' s~ed~imnt samnples

Page  24 24 Q I zS.. -' -P Figure 10 Phi. Madian distribution

Page  25 25 feldspars and various heavier minerals making up the remainders. They are typical of sediments derived from the glacial drift. Except at its center Sleeping Bear Shoal is seen to be covered by a rather uniform medium sand with a strip of fine sand along the shore of Sleeping Bear Hill. The sediment on the slopes both north and south of the shoal is fine sand containing up to 15o silt. A band of medium sand extends around the tip of Sleeping Bear Point and along the south shore of Sleeping Bear Bay. The remainder of the bay, except for the cobble area on the east side, contains fine and very fine sand with increa-ing silt content northward. The floor of the central basin is covered with a medium silt which contains about 2$^ sand and less than 5, clay. Tho sing-e line of samples which extends from the central basin northeast to 'or th Iaanitou Shoal Lght showCs an _abrupt change from th e silt of the basin to a rediium sanc. Tne sand in the deep channel between Pyramrid Point and T.orth!Tanitou Island is coarse and contains 3ess sil.t than the sand at corresprondincg dc eth, in the leepi ng Aear Point area..Thle single sample of sand on the top of ortibh.anitou Shoal is somewhat coar.ser than any found on Sleeping e-ar Shoal. Phi deviation is a rgeneral measure of the degree to which a sediment i; sorted. The numerical value of the deviation beconmes smaller with progressively greater degrees of sorting.. Fi.ire 11 is an isopleth plot of the deviation of the sediment samples. The sand on the shoal areas is seen to be quite well sorted and tl.-t of the slope arelas somevlwhat e s:n so. It i. sig:nificant to note that the most poorly sorted imaterial is found on anrd nar the ba:.e of the slope off the tip of Sloeping Bear Poin t. This iJ probably partially due to the addition of wind-blowrn,-and which wan,; obuoervo.d -to bo cartriedo out to that.ae.

Page  26 26 k 0. 0 —...?. 0 1 -,O o.5, W"Z —N 0 -4 0.00 Figu~re 11 - Phi Deviation distribution -

Page  27 CUL.tREITS Sleeping 3ear Shoal The current meter developed for these investigations has a sensing unit which consists of a pendulum suspended from the apex of a tripod by means of a univers:al joint. At the lower end of the pendulium there in a high-c3dra configuration consisting of a vertical cylinder 11 inches in diaimeter by 10 inches high, open at both ends. It is deflected from the vertical in the direction of current movemnent. The amount of deflection i- a function of the current velocity. The orientation of the po'ndul.u iS sensed by mercury switches contained in its upper end. The switches are connected via a.flexible multiconductor cable to a recording unit. For the 1963 installation, the recording unit was battery operated a-nd housed in a waterti-ght container placed on the lake bottom near the tripod. This unit made a spot recording, of the current direction and velocity ra^e-c every 20 minutes by disch!arging capacitor;- through a strip of electrically sensitive pap er. The meter wa: calibrated in the tow tank facility cf the lXpartment of Naval Architecture and:Marine tEginoering at t'h University of laichigan. 'Te velocity ma.gnitudes recorded by this model, were: 1, 8-1.5 ct./sec; 2, 15-23 crn/cec; and 3, over 23 ca/orec. Since the drag portion of the pendulum,. cover: a considerable vertical interval which i3 directly in the zone of maximum current shear, these velocity magnitudels can only be con:idolred to be approximationo of the general water movement near the bottom. 27

Page  28 28 The meter was placed in 20 feet of water about 1200 feet off the west face of Sleeping Bear Hill (fig. 5). Unfortunately, a progressive malfunction of the paper advancing mechanism led to an increasing irregularity of event timing and finally complete failure after about two months of operation. For the first month of record, however, it was possible to record observations representing about 95~ of the elapsed time. Even though the time of each observation could not be established except at the beginning and at each time the chart paper was changed, it was at least possible to record the frequency of occurrence of the various current directions and velocity ranges. The period, of good record ex'tends from 1200, 8 August whlen the meter was first emplaced until 1600, 4 September 1.63 when the chart paper was changed. This period appears to be representative of almost the entire range of current. velocities since it includes both call. periods arAd a severe st:orn which occurred on 12, 13, and 14 August with sustained winds of over 50 mph. Unfortunately the current velocity for much of this period exceeded the hiheoat range of the meter so that the true maximumn velocity is not known. Only the tr::-c -'re late atfal s1) __s would be expected to produce more rigorouos,:n.; ':.ao;3s. Table 1 lists the frequencies of the various currents; at this point and fiagre 12 is a rose showing this same inforimation. ~ote that the currents are named according to direction towalrd which they flow. It is readily apparent thu thth current flow is almost all directed north and northwestward. This can be most readily explained by considering the proximity of the shoreline to the east of the meter site. Whilo working at the::ite, divors noted that the surface currents under

Page  29 29 Direction of Current Flow EE E S S SW W 'W N totals 8-15 cm/sec 2.5 8.5 19.5 30.5 15-23 " " 1.0 0.5 2.5 3.0 6.5 27.0 40,5 23 " " o.5 8.5 3.5 5.0 2.5 20.0 totals 1.5 0.5 11.0 9.0 20.0 49.0 Periods of little or no current 8.5 Table 1 Current Frequency of Sleeping Bear Shoal 8 August through 4 September 1963 Expressed[ as percentage of the total observations that the current flowed toward the direction indicated. the prevailing southwesterly winds were generally flowing shorevward while the bottom currents at the same tirme flowed northward and northwestward in an offshore direction. his could be explained by assuming the existence of a set-up of the lake surface against the shoreline with consequent secondary currents:flowing down the gradiont of the set-up and being deflected to the right by Coriolis force. This general process is described by Ayors (1962). iTorth },anitou Shoal Since the results of the 1963 current m easurements matde it appear desirable that a better time relationship between winds and currents be established, the current meter location was moved in 1964 from the open lake off Sleeping Bear Iill to the vicinity of the North Tainitou Shoal Light Station (marked 'Light' on figure 2). At this location

Page  30 30 North - 50o 40% 30o 20% Figure 12 Current rose for Sleeping Bear Shoal 8 August through 4 September 1963

Page  31 31 wind direction and velocity wore sensed by a wind vane and anemometer mounted on the top of the light struoture about 100 feet above the water surface. The sensors were connected to a recording mechanisi;., located in the living quarters of the light station,which was tended by the U. S. Coast Gard pers.onnel who manned the station. The current meter was placed 400 feet north of the light structure and connected to the same recorder by cable (fig. 13). Unfortunately the limited availability of multi-conductor cable at that time m-.e it necessary to restrict the reporting of currents to four velocity rar-es and only two directions. The record obtained yielded hourly ruammaries of the following: wind direction, average velocity of the wind (recorded in mph and converted into Beaufort force), general current direction (eastward and westward), and approximate magnitude of the current velocity. For the 1964 season, the. high drag configuration on the penduluru was redesigned in order to decrease the sensitivity of the meter. The new coifj,-lr.ttion was in the form of a rhombic dodecahedron about 10 inche. across.. Recalibration in the university tow tank facility yielded the following current velocity magnitudes: 1, 25-40 cm/sec; 2, 40-50 cm/sec; 3, 50-75 cm,/sec; and 4, greater than 75 cm/sec. Wind records were obtained starting 2300, 5:Iay through 1600, 5;ovember 1964 for a total of 4409 hourly observations. The recording systemr was not operatin, for a total of 82 hours of this time. The wind record is, therefore, 98.14 complete. The current meter was placed in operation at 2100, 30 July and operated until it was overturned by wavo action during a storm at 1800, 26 September 1964. This period totals 1387 hours and the combined wind-current record is 97.27o complete.

Page  32 3 2 -4 "I ,:I- r. e l'i t., C- r 1 ` -,) 2 a c o '- - `- r i i - o -,,, - - 0 II I I..c"',.., A 0 - I

Page  33 33 The initial step in analyzing this information was to determine the frequency of occurrence of the various wind directions and velocity ranges for the entire period of wind record. In order to simplify analysis the wind velocities were grouped into three ranges: Liildwinds, force 1-3, 1-12 mph; Strong winds, force 4-6, 13-31 rmph; and Gale winds, force 7-9, 32-54 mph. This grouping has a practical significance for this study because, in general, it was only possible to make collections and current observations during periods of mild winds. The information is presented as table 2 and figure 14. Lote that the winds are named according to the direction fromr wh-ich they blow. The salient characteris'tic of the wind patternl is seen to be that whereas there is no domrinant direction for mild winds, there is a great predominance of stron., and gale winds from the south w.est. In the same manner, a wind frequency tabulation was made for approximately the same period as the available current record. The Wind Direction IN B S. E S SW W 1 7- totals 1-12 mph 6.0 3.9 6. 3.5 5 52 5-0 4.9 5.1 39.8 13-31 mph 2.3 0.9:.7 8.1 22.1 5.2 6.9 7.2 54.5 32-54 mph 1.3 3.3 0.7 0.4 5.8 totals 8.4 4.9 7.8 12.9 30.6 10.9 12.2 12.3 Table 2 Wind frequency at 1Iorth ianitou Shoal 5 M1ay through 5 NoovNember 1964 Expressed as percentage of the total observations that the wind blow from the direction indicated.

Page  34 34 North 20O 3o% Figure 14 Wind Rose for North Manitou Shoal 5 k y-thr>ulgh 5 November 1964

Page  35 35 tabulation covers the period 26 July through 26 September 1964 (table 3, figure 15). There is no substantial difference between the wind regimes of the two month period and the entire six month period. IXuing the period of wind and current record, the wind through the passage blew from the westerly quadrant 55% of the time and from the easterly quadrant 21% of the time. Since it was only possible to record the current data on a basis of a generally eastward or generally westward flow, the frequency of the various current directions and velocities can be depicted as a simple bar histogram. (fig. 16, table 4). The hourly plot of winds and currents shows that the current generally flows toward the direction from whi.ch the wind is blowing. It is immediately apparent that there is a major difference between the wind and current regimes. The current flows toward the west 54% of the ti1me which is almrost exactly the time fraction during Wi.nd Direction BN E S S SW W 'VW N totals 1-12 mph 5.6 3.5 7.8 3.1 5.2 6.3 5.5 4.0 41.0 13-31 mph 2.2 2 2.0 22 8.8 19.0 8.3 8.5 4.4 55.5 32-54 mph 0.1 1.3 1.5 0.6 3.5 total. 7.9 5.6 10.0 13.2 25.7 15.1 14.0 8.4 Table 3 Wind frequency at Itorth.'anitou Shoal 20 July through 26 September 1964 Expressed ns percentage of the total observations that the wind blow from the direction indicated.

Page  36 36 North / 10 20% igure 15 Wrind ro.3o for North:Manitou Shoal 26 July through 26 September 1964

Page  37 37 Westward Eastward - 50% -40% - 30% 20% _-. -- — J_- 1 — O t C' vlo 0 0 u'0 0 o'-N Ad Ja V A d A Velocity Ranges cm/sec A 0. u' Figure 16 Current frequency histogram for North Manitou Shoal 30 July through 26 September 1964

Page  38 38 25-40 cm/sec 40-50 cm/sec 50-75 cm/sec 75 cm/sec totals General Current Direction WVestward Eastward totals 15.4 1.9 17.3 26.7 9.6 36.3 11.9 11.8 23.7 0.1 2.2 2.3 54.1 25.5 Periods of little or no current 20.5 Table 4 Current at a orth Manitou Shoal 30 July through 26 September, 1964. Expressed as percentage of the total observatiorn that the current flowed toward the direction indicated. which the i.nd blotws from the west. Likewise, the time fractions of eastward flowing currents and winds blowing from the east are aino st identical. nTe inescapable conclusion is that the current flow past the meter must be contrary to the wind direction for a substantial portion of the time. In an attlcmpt to find a possible eqxlanation for this unexpected situation, an- hour-by-hour plot of the wind and current was made im graphical form. The only definite relationship which it is possible to obtain from this plot is that a gale force wind from the s'outhwest will cause an eastwardi flowingg current at the nmter site. Under all other wind directions and velocities, the current fluctuated btetween easttward and westward in an unpredicta ble mannor. Currents often remained steady under shifting wind.j and juost as often fluctuated while the wind romaineod r toady.

Page  39 39 There are several possible reasons for these apparently inconsistent results. The first possibility is that tho current recording system might have malfunctioned. However, the individual switchand-circait nature of the instrument insures that the device either operates correctly or does not operate at all. There remains the possibility that the device was oriented incorrectly on the lake bottom thereby reporting eastward currents when they were flowing toward the west. This is rendered unlikely by the orientation checks made at the time of installation. The tripod alignmrent was checked both by diver's compass and by reference to the line of the cable extending from the meter to the light structure. Frther, it was observed that the meter reported a strong eastward flowing current just prior to its being ove:rturned during the gale of 26 September 1964. Whein it was recovered on 5?>ovember 1964 the tripod was found to be overturned toward the east. Terefore, the meter must have been registering the correct current direction at the time it was over turned.. A second possibility is that the current at the rmter site is typically stratified, with t;he surface current flowing in direct response to the wind and the bottom current representin.g a counterflow. This condition does e-xit at the 3.963 meter site on Sleeping Bear Shoal, which is quite near the shore. Mowever the 19?4 meter site is over 3 miles from the nearest poinl of. and and in a very exposed position. Any condition of sot-up and counter-flow would be unlikely. The shallow depth at the site precludes any significant thermal stratification during tho late surmer period of record. At no time during the ontiro sulrmor and fall was a stratification

Page  40 40 observed by divers, nor did they ever find the bottom current to be significantly different from the surface current. If the two above possibilities are discounted it becomes necessary to assume that the current in YTanitou Passage cannot be a simple through —flowing current which responds directly to the local winds, but must have a more complex interrelationship with the rest of Lake Michigan. This could manifest itself as the observed current pattern in either of two ways. The passage might have an unstable current regime with a complex of s:hifting gyrals causing repeated fluctuations of the current at anr point. On the other hand, conditions might be such that the current at the 7Torth tanitou Shoal meter site flows northw-ard or southwardl for a significant fraction of the time. The restricted direction-reporting ability of the meter wou1ld result in its reporting the full velocity of a north or south current but it would report whatever slight eat t or west comiponent that current might have a5 h full indication.of east ward or we:stward current. It will require a muclh mrore extensive survei of the cturrent patt-,ern of the Manitou Pa:;sao area before this problem can be resolved. The t)abulation of current velocitie.i (table 4) i. however, quite valid and indTicates an extremely vig;orous current regime. At tlhis3 site, currents strong enoughl to erode zand trn..n3port medium sand exiot 804 of the time and currents powerful 1enoug-h to erode silt, sand and fine gravel are attained 2,1 of the timo. Assumptions.; of the ability of a lvoen current velocity to erode a sediment are based on the Threshold.'oan Velocity curve of Hjjuls.treo-n as describedl by Tnrran (1963). This uoses the rmoan current velocity measured 100 cm abovo the bottom.l Since the height of the lower eil,

Page  41 41 of the pendulumt varies frolm about 20 cm to over 50 cm, thi; approximaation will yield conservative estimates. CO"CLUSIONTS Despite the inability to describe the current pattern in Yianitou Passage, it is evident that the near-bottom currents over the shoal areas and along the shoreline are often capable of. actively eroding and transporting the sediments found in these areac,. I'oreover the land adjacent to these areas consi;.ts of a sandy moraine material which i; very easily eroded (Kelley, 1957). During high water stages of Lake Michigan the bluff shoreliines of the area, such as the vwest face of Sleeping 3ear Hill are subject to direct wave attack and'the vigorous alorongrhore currents certaJinly rmlu,;t be a)le to remove te.aterial a; fas:t as it ii eroded, Gillis anr! Bkeman (1963) report that they measumre a retreat of the crest of the bluff on the wesOt face of Sleepinrg Dear Hill of 53 feet in 30 years (figs. 1 and 17). Since this slope is close to the angle of repose for thio material the figire rmust indicate the re-treat of the entire face of the hill]. If the overall rate of retreat of the shoreline lht been at about this rate, the hill iiuct have extended over a mile farther into the la.e at the beginninrg of the }ipisln,;r; tage. About a niile 'outhward from the tip of S1leeping Bear Point a layor of cobbles is found e:xpo;ed in, the face of the low hill which lies just inshore from tho belach. Tho material above thi layer is clean medium sand whilo belovw it i; a reddis:h brown sand very similar to the material found. uncder the c-obblo areol at the center of Sleopinri

Page  42 ,42 - I S 3jl 1SM jUre 17t Prof ile o of the, wes t fla ce- ofA Sl e cpainj;.2~0ear Ilill

Page  43 43 Bear Shoal. This layer is about 10 feet above the present lake level which means that it would have been at a depth of 15 feet during the iipi3ss-3in stage and could well have been an offshore cobble pavement area similar to those found on the shoal areas now and that it has been subsequently covered by beach or dune sand. This implies a major outbuiltdiin of Sleeping Bear Point in the last 4,000 years. The above mentioned retreat of the face of Sleeping Bear Hill permits another es,..;;.,-,- of the approximate intensity of the sedimentary activity of tne area. If this 50 foot retreat of the slope is assumed to apply to the whole length of the weot face of the hill (about 10,000 feet) this would imply the removal of some 1.50,000,000 cubic feet of material in theo 30 year period. If only half this rrat.erial were carrled around, or blown over, Sleep ing; Bear Point, it would be enough to cover the we-ftern half of Sleeping De-ar Bay with a layer about a foot thick. ThiS.gr agrees quite well with the previous estirmate of 2-3 feet per century. Considering the demonstrated vigor of the ogological processes at work in the:Tanitou Pas.;agE areta, it becomes pos:;ible to imake: an estim:.ate of the recent geortmorphic, his jtory of the area as follow1:1 Sleoping Bear Hil3 once ex tended overm tuch of the present area of Sleeping rear Shoal. he hill har; been cut back by wind and water action and. its material tran.:slported northwa.rd. lich of this material han gLone to form Sleeping Boar Point with its numrerous sand dunes. The rest of the sand ha, boon carried around Sleeping Bear Point to fill part of Sleeping Bser J:a3y ond isolate the present Glon Lake from Lakeo lichigan (Johnson, 1958).

Page  44 44 South llanitou Inland is being moved eastward, the western side being eroded, and transport of sand around both the north and south sides of the island is constructing the opposing points which enclose the natural harbor on the east side of the island. Much of the southern end of ITorth Manitou Island has been removed and its material may have been dropped into the deep area to the east of the island. In a similar manner the tip of Pyramid Point has been remove d. The central basin has remained a body of water throu,'ghout postglacial time and is at present receiving the silt fraction of the eroded sediment t over its entire extent while being encroached upon by the advancing Sleepirng ear and South 1anitou Shoals. OWving to the extreme rapidity of geomorhi c evolution in Tanitou Pas;age, the r aeffords an excellent opportunity for the observation. and study of the processes of ero;ion and deposition. in the necarshora and offshore environmlents.. Furth'er extensive and intersive situy of the area should yie l knowledge applicable to imuch of the remaimnder of the Creat Lakes.

Page  45 APPETTIDIX M.Kechranical composition of sediment samples (Locations aro sho~vri in fijure 9) Phi Measures, Median Ye an Doviation Sar I e 3Skewness1 000 001 002 003 004I 1.8 2.8 -3.1I 1. 9 2.0 I I 005 006, 007 009 01 0 013I 0-13 014 015 017 0149 020 021 023 024 027 028 029 030 031 032 2.0 3.1 2.0 1.1 1.7 2.2 1.7( 2. 2 2.7 1.7 2.3 2.0 Pebbe s and C:1l-1 cobbl-es 1.9 16 2. 5 2. 4 3.1 3.0 17 1.8 2. 8 3. 2 2.8 2.6 2. 8 2. 9 2. 1 1. 1 1.8 2. 2 1.8S 2.8 2. 6 1.7 1.9 2.3 2. 1 1.4. 1.5C 2.0 1. 6 1. 9 2.4 1.9Q 2.4 3. 2. 3.4 3. 2 1.6S 0.4.1.2 1.8 1.4 1.4 1.4 1.0 0. 6 0. 6 0.5 0. 6 0. 6 1.4 0. 8 '0.,4 0. 6 0.5 cobbles 0.4 0.5 0.4 0.4 0.3 0.4 0. 6 0.4 0.7 1.-7 0.6 0. 8 1.0.06.64.57.57 -.20.17.20 17.43 -.13.20 -.20 -. 25 *.25 -. 17 25 -.12.50.25. 10 45

Page  46 46 Sample Phi Measures No. Median lean ADviation Skewness1 100 1.6 1.6 0.3 101 2.1 3.1 1.8.56 102 5.4 3.8 2.0 -.80 103 5.8 4.9 1. -.64 105 1.8 2.7 1.4.36 106 2.5 2.7 1.4.14 107 Small pebbles 108 1.5 2.1 1.2.50 109 2.0 2.9 1.6.56 110 Cobbles 111 3.6 3.7 2.2.05 112 2.9 3.5 2.2.27 113 2.0 2.7 1.5.47 114 1.9 2.1 0.7.29 150 1.7 2.0 0.7.43 151 1.8 2.0 0.7.29 152 1.8 2.6 1.3.62 153 5.? 4.2 1.8 -.56 154 5.8 4.6 1.9 -.63 155. 5.4 3.9 1.8 -.83 156 2.9 2.9 1.3 157 3.5 3.5 2.0 160 1.3 1.3 0.5