THE MAESEILLES MORAINE. 315 



INNER-BORDER TILL PLAIN. 



On the inner border of the Marseilles moraine from northern Kendall 

 County, where it separates from the Minooka Ridge, southward through 

 Grundy and adjoining portions of Lasalle and Livingston counties, and 

 thence eastward through Kankakee County, there is a till plain which 

 shows a perceptible descent away from and nearly at right angles to the 

 moraine. As the moraine describes nearly a half circle in this interval, the 

 sloping plain converges toward a focal point, which is located near the head 

 of the Illinois River. There is a similar descent toward the head of the 

 Illinois from the east; thus a basin is formed, which is commonly known as 

 the Morris Basin, from the city of Morris, which stands near its central 

 portion. The slopes of this plain are well shown by the direction of drain- 

 age lines, which converge from all quarters toward the head of the Illinois 

 River. (See topographic map, PI. III.) At present the basin is deeply 

 notched at its western border by the valley of the Illinois River, but at the 

 withdrawal of the ice sheet the evidence is quite clear that the western rim 

 stood sufficiently high to hold a lake of considerable size in the Morris 

 Basin. The history of this lake is discussed in some detail below. 



On the border of this till plain next to the moraine the surface is 

 gently undulating, but within a few miles it become an almost expression- 

 less plain. In Kankakee, southwestern Will, and eastern Grundy counties 

 the plain is occupied by sand dunes and beaches; elsewhere it is generally 

 very smooth. There are, however, a few points in southeastern Kendall 

 and northeastern Grundy counties where the drift is slightly ridged. 



In the vicinity of Lisbon, in southern Kendall County, the drift is 

 insufficient to conceal the inequalities of the limestone ridges, but the 

 surface is only gently rolling, the crests of the ridges being scarcely more 

 than 20 feet above the sags. This area is 2 to 2 J miles from north to south 

 and 5 to 6 miles from east to west, and lies mainly in the south half of 

 T. 35, R. 7 E. 



The drift along the border of the Marseilles moraine has generally a 

 thickness of 100 feet or more, but upon descending the slope toward the 

 head of the Illinois the thickness decreases, and there are extensive areas 

 in eastern Grundy, southwestern Will, and northern Kankakee counties 

 where rock is encountered at veiy slight depth, so that the shallow ravines 



