224 PLEISTOCENE OF INDIANA AND MICHIGAN. 



STRUCTURE OF THE DRIFT. 

 THICKNESS. 



On the borders of Lake Michigan the drift extends considerably below lake level, except in 

 a few places such as the Waverly district near Holland and the New Troy district in Berrien 

 County, where rock is reached a little above lake level. At Ludington and Manistee the drift 

 extends below sea level, about 640 feet being present in some of the borings for salt on the low 

 plain at these cities. Bedrock surface is only 300 to 400 feet above sea level along much of the 

 eastern shore of Lake Michigan between Grand Haven and Grand Traverse Bay, and is generally 

 less than 500 feet above in the district south of Grand Haven, as may be seen by reference to 

 the map of rock contours (PI. II). The altitude of the rock surface inland from the lake is 

 known at very few places, but it probably nowhere much exceeds 800 feet above sea level in 

 the northern end of the southern peninsula. Its maximum height may be attained in the 

 northern part near Gaylord, but it is very doubtful if it is held beneath the highest part of the 

 morainic system hi Wexford, Lake, and Newaygo counties; more probably the rock there is as 

 low as the level of Lake Michigan. Such being the case, the drift on these high ridges is 700 to 

 1,000 feet or more in thickness and it probably averages over 500 feet from the vicinity of Gay- 

 lord to Newaygo. From Newaygo to the head of the lake the average thickness is probably 

 not far from 200 feet, the altitude of this district being comparatively low and the rock surface 

 somewhat higher than in the district to the north. 



The drift is probably in large part pre- Wisconsin in age, but data are not numerous that bear 

 upon the separation between the Wisconsin and pre-Wisconsin deposits. Records of flowing 

 wells in the district about 4 miles southwest of Shelby in Oceana County indicate the presence 

 of peat or black muck at a depth of 175 to 200 feet (75 to 100 feet below the surface of Lake 

 Michigan), and this seems likely to mark the separation between the Wisconsin and pre-Wisconsin 

 drift. The wells were continued only to the bed of sand or gravel which immediately underlies 

 the black muck. One well penetrated 15 feet of muck and two others nearly as much. The 

 older drift beneath the muck is likely to be fully as thick as the younger drift above, for a well 

 at Shelby, 4 miles away, on ground only about 100 feet higher than at these wells, penetrated 

 500 feet of drift. 



COMPOSITION. 



Along the entire length of the rugged portion of the Lake Border morainic system wells 

 ordinarily pass through a large amount of loose-textured sandy or gravelly drift. In places this 

 material is thinly capped with till but is thickly set -with bowlders; in many other places it is 

 sandy from the soil down to the bottom of the wells, most of which require curbing while in pro- 

 cess of excavation. Bowlders are conspicuous on the prominent ridges from Oceana County 

 northward. Large limestone blocks also occur; some of these in Oceana County were burned 

 for lime in the early days of settlement, and led to the incorrect inference that limestone ledges 

 are present in that county. Wells, however, indicate that the rock surface lies nearly 300 feet 

 below the surface of Lake Michigan. Calcareous material, which is found in considerable 

 abundance in the loose-textured drift deposits of the region, as noted many years ago by 

 R. C. Kedzie while analyzing soils from that part of the State, 1 gives the soil great fertility. 



From Muskegon River southward to the head of Lake Michigan the ridges of this morainic 

 system are composed largely of grayish-blue clayey till. In some places the till contains only a 

 few small pebbles, and in others it is laminated, indicating that it was water-laid. The crests 

 of the ridges, however, seem to be land laid. Exposures of the laminated material may be seen 

 here and there along the bluffs of Lake Michigan from Kalamazoo River southward. 



Bowlders are found in moderate number along these till ridges but are less numerous than 

 on the rugged portion of the morainic system farther north. They are chiefly of granite, though 

 other kinds of rock are present. Large bowlders of reddish sandstone, similar to those on the 



i Ann. Bepts. Michigan Board of Agriculture for 1878, 1887, 18S8, and 1893. 



