STRUCTURE OF THE DRIFT BORDER. 65 



the ice sheet reached into northern St. Louis County from the Illinois side 



of the river. 



On the east bluff of the Mississippi below East St. Louis only a small 

 amount of glacial drift has been found beneath the loess deposits, which 

 there cap the bluff to a depth of 30 to 50 feet or more. The drift usually 

 consists of a thin bed of stony material, but in some of the recesses of the 

 bluffs and in ravines exposures of nearly pebbleless clay are occasionally 

 seen. Some of these exposures near Columbia, in Monroe County, reach a 

 depth of 40 to 50 feet. An occasional bowlder a foot or more in diameter 

 is found in these deposits, but stones are very rare compared with their 

 number in the typical till, such as is exposed in the east bluff of the Missis- 

 sippi above East St. Louis. It is probable that the ice sheet extended as 

 far west as the east bluff of the Mississippi in St. Clair, Monroe, and Ran- 

 dolph counties, but the deposits there are very much thinner than in drift 

 ridges, discussed later, which traverse the eastern portion of these counties, 

 and which perhaps mark an ice margin at a somewhat later period than that 

 of the maximum extension. 



The portion of the drift border in southern Illinois, on the slopes of 

 the elevated rock ridge in Jackson, Williamson, and Saline counties, con- 

 tains typical till, but the deposit is seldom more than 20 or 25 feet in depth. 

 In one instance, however, a well in the southwest township of Williamson 

 County was found to have reached a depth of 70 feet without entering rock. 

 The distance to rock is also great in the vicinity of Murphysboro, in Jack- 

 son County. The wells and borings for coal often reach a depth of 100 

 feet, and occasionally 130 feet, before entering rock. The drift is reported 

 to be largely sandy material, but the upper portion, exposed to a depth of 

 50 feet by Big Muddy River, is mainly clay containing but few pebbles. 



On the borders of the Ohio Valley, in Gallatin County, Illinois, there 

 is a belt of sandy material several miles in width which is not referred with 

 certainty to glacial deposition. Back of this a typical till sets in, which is 

 exposed in ravines beneath 8 or 10 feet of loess. Wells usually reach the 

 bottom of the drift at 20 or 30 feet, but one 2£ miles north of Ridgway 

 reached a depth of 98 feet without entering rock, and another 3 miles west 

 of Ridgway a depth of 75 feet. In the Ohio Valley at Shawneetown a 

 boring for gas and oil penetrated 112 feet of alluvial and other deposits 



MON XXXVIII 5 



