290 PLEISTOCENE OF INDIANA AND MICHIGAN. 



Along the shore of the "thumb " the older till begins to outcrop about 6 miles north of Lexing- 

 ton. The exposure here is small, but it evidently passes below lake level to a considerable dis- 

 tance. Along the shore 2 miles north of Richmondville it appears in greater thickness and is 

 typical in color, hardness, and content of striated stones, and in the firm manner in which the 

 stones are held, even after the till around them has been largely removed. (See PI. XII, A, B.) 

 It is almost as resistant to wave erosion as the shale that outcrops on the same shore but is cut 

 into deeply along its joints. (See PI. XII, A.) 



On the shore at Forestville a nearly vertical cliff about 50 feet high consists of old till and a 

 few feet of pebbleless lake clay of the finest texture, known locally as ' 'polish." This lake clay 

 appears to belong with the older till and not with the newer till above. 



At Croswell the older till is exposed in the banks and bed of Black River. At the west end 

 of the bridge south of the village it forms a rocklike wall, and a little farther south it is overlain 

 by ferruginous gravels. It may be seen along the banks of Black River for many miles below 

 Croswell. West of Amadore over 30 feet of it is exposed in the west bank of the river. 



As already noted (p. 261), it is exposed in a number of places in Tuscola County and appears 

 in most well borings with a thickness of about 100 feet. West of East Dayton its surface is 

 exposed in the base of the hill, where it gives rise to a wet belt or line of springs. 



A mile and a half south of Avoca the old till with a distinct old soil at its top is well exposed 

 in the north bank of Mill Creek beneath 28 to 30 feet of the newer till. (See PI. XIII, A.) 



The greater part of southeastern Michigan is deeply covered with drift and has few exposures 

 of bedrock. On this account, mainly, glacial stria? have been found in very few places. Partly, 

 however, their lack is due to the fact that shales of the softness of those in this region seldom 

 retain stria?, the principal rocks on which they are recorded being limestone and less commonly 

 sandstone. 



There are finely developed stria? and groovings at the Sibley limestone quarry a mile north 

 of Trenton, and Sherzer reports stria? at several other places in Monroe County. 1 In the Sibley 

 quarry Sherzer distinguishes three sets of stria? and a few broad, shallow troughs which he 

 ascribes to an ice movement earlier than any of those that produced the stria?. In one of the 

 principal troughs he finds the stria? related in such a way as to show in his opinion their relative 

 age or order of making. He concludes that they indicate two ice invasions from the Labrador 

 center. To the writer it seems probable that all the stria? at the Sibley quarry were made by the 

 last ice invasion, but that the troughs are probably older. 2 The rock ledges near Trenton stood as 

 a reef in the axis of flow of the Lake Huron ice lobe during the maximum of the last ice sheet and 

 during a considerable time before and after the maximum, or so long as the ice from the Lake 

 Huron basin pressed southward some distance beyond Trenton. The troughs, however, may 

 not be due wholly to ice work. Still, the position of the Trenton ledge with reference to ice flow 

 from the basin of Lake Huron and of the troughs running back from the struck edge of the ledge 

 is exactly like that on Kellys Island, in Lake Erie, with reference to the ice flow in that basin; 

 and the troughs may therefore have been largely due to the ice work. The principal trough 

 (S. 38° W.) described by Sherzer and the first two sets of stria? (S. 31° W. and S. 68° W.) are 

 evidently due to ice coming from the Lake Huron basin, and the last set (N. 29° W.) is due to ice 

 flowing out of the Lake Erie basin. 



The passage of the Erie ice over the Trenton reef could occur only when the Huron ice lobe 

 ended some distance north of Trenton, probably at the Detroit interlobate moraine, which runs 

 south from Birmingham through Detroit and thence southeastward nearly to the shore of 

 Lake Erie at Leamington, Ontario, where it turns toward the east. Only for a relatively brief 

 time would this peculiar relation of the two ice lobes exist, but it would occur both in the 



i Sherzer, W. H., Geological report on Monroe County, Mich.: Michigan Geol. Survey, vol. 7, pt. 1, 1900, pp. 128-132, map opposite p. 112. 

 The striae near Trenton are discussed in Sherzer's paper on Ice work in southeastern Michigan: Jour. Geology, vol. 10, 1902, pp. 194-216; and more 

 fully in his report on the geology of Wayne County: Kept. Michigan Geol. and Biol. Survey, 1911. 



2 In May, 1913, Mr. Leverett visited this quarry and found the order of development of the three sets of strise to be as follows: (1) S. 31° W., 

 (2) S. 68° W., and (3) N. 29° W. 



