270 PLEISTOCENE OF INDIANA AND MICHIGAN. 



Between the Arkona beaches and Cass River a large share of the surface is covered with fine 

 sand. The Arkona beach ridges are composed of gravel, but the Warren beach is generally 

 sandy. 



Borings have not disclosed any thick or extensive beds of gravel or sand beneath the sur- 

 face sheet of till. Almost invariably they find the whole depth to rock to be occupied by till and 

 hardpan or old till (see p. 261), with only a few thin layers of sand or gravel. The drift in this 

 district strongly contrasts in this respect with that of the high region in the northern part of the 

 southern peninsula, where in many places a relatively thin coating of till covers great depths of 

 gravel or sand. 



THICKNESS. 



In Lapeer County the thickness of drift ranges from 50 to 200 feet, with an average of a 

 little more than 100 feet. In the morainic belt in southern Tuscola County the thickness is 

 greater, but on the flat plain to the north and on the plains in Sanilac County it ranges from 20 to 

 120 feet. At Brown City near the Lapeer County line two wells pass through 190 and 213 feet 

 of drift. In northwestern St. Clair County the thickness varies from 150 to 225 feet. Some 

 of the deeper borings probably show the location of buried valleys in the rock surface. 



TILL PLAINS. 



Till plains are for the most part very broken and patchy. This arises from the confusion 

 and overlapping of the moraines in the interlobate area. The plain in Bich Township, Lapeer 

 County, is perhaps of till, but is covered to some extent with lake clays. A few small till plains 

 occurring in the midst of morainic deposits are quite remarkable, because of their striking con- 

 trast to the moraines; such are the one west of Columbiaville, a smaller one to the north, and a 

 small one between Mayville and Silverwood. 



The most important till plain of this district, however, is that which occupies a large part of 

 west-central Sanilac County and runs over into Tuscola County in Novesta, Kingston, Ellington, 

 and Wells townships. It lies northeast of the morainic belt, west of Black River and Elk 

 Creek and south of the interlobate district in northwestern Sanilac County, covering 450 to 500 

 square miles. It contains only a few small fragmentary morainic ridges and a few eskers. Its 

 altitude is mostly between 800 and 850 feet. The drift is relatively thin, varying generally 

 between 20 and 50 feet, is less stony than that of the moraines, and is not covered with lake clay. 



Other smaller plain areas of till lie in southern Sanilac and northern St. Clair counties, but are 

 considerably broken by the moraines and transverse ridges. 



LAMOTTE ESKER. 



A rather remarkable esker 11 miles long stands on the till plain of western Sanilac County. 

 For about 6 miles it runs south in the western tier of sections of Elmer Township along the medial 

 line of a swamp that follows the south branch of Cass River. In this part the esker, which is 

 about 30 feet high, is broken into several pieces, but they are ridge forms and are aligned after 

 the manner of true eskers. In some respects, however, the ridge is not a normal esker, for the 

 fragments contain a considerable amount of till and in places many bowlders. The fact, how- 

 ever, that they are composed also largely of gravel and sand, taken in connection with their form, 

 alignment, and relation to the swamp, seem to show that they are in reality a modified esker. 



In sec. 6, Elmer Township, the ridge (tracing it headward) turns sharply northwest and at 

 about 2 miles divides in two branches, one continuing northwest for about a mile and the other 

 running directly west for over 2 miles and turning northwest at its end. This east-west section 

 is a more typical esker. It is more generally composed of gravel and sand and one or two of its 

 knobs rise 60 feet above the surrounding plain. 



Another ridge about 3 miles long crosses the county line a mile south of Novesta and 

 extends southeast toward the Lamotte esker. Its gravelly knobs seem more like kames 



