THE GLACIAL PERIOD 63 



hills and planing down their tops, of gouging out valleys, and of 

 transporting incalculable loads of debris. 



Gradually the rigorous climate of this early period was 

 mollified. Spring came earlier as the years passed and the 

 autumn days grew warmer, so that the glacier beat a slow retreat. 

 As the front slowly melted back through many, many years the 

 rock and soil debris held in the ice was deposited all over the 

 smoothed and scored rock surface in an unsorted condition, 

 the finer material containing subangular rocks and bowlders of 

 all sizes, polished and scratched. It is the same sort of drift 

 that makes up the terminal moraine but now constitutes the 

 ground moraine, the deposit laid down by the retreating glacier. 

 This withdrawal of the ice was irregular; it would fall back 

 only to advance again as a few cold winters reinforced its reserves. 

 Its front was constantly shifting as melting or temporary advance 

 went on at some points more rapidly than at others, due to local 

 conditions. The deposited drift was therefore left in erratically 

 arranged heaps with irregular hollows between. The drift some- 

 times buried great blocks of ice which later melted, leaving de- 

 pressions. Most of the valleys were without outlets except as 

 chance arranged them, so that later they were occupied by 

 lakes, ponds, bogs, marshes, and swamps. 



This Illinoian ice age was followed by a long interval when 

 the surface of the soil was subject to erosion; when in all prob- 

 ability forests developed and grassy plains and valleys lay 

 luxuriant in the warm sunshine. Other ice sheets again invaded 

 the region, however, bringing desolation. In these later ages, 

 the lowan and the Wisconsin, the ice in the Chicago region was 

 nowhere nearly as thick as in the case of the Illinoian. These 

 later ice sheets apparently over-rode the early drift deposits, 

 piling up their moraines and other deposits upon the earlier ones. 

 The average depth of the drift in the Chicago region is probably 

 130-140 feet, as determined by wells, with a maximum thickness 

 of somewhat over twice this figure. Most of this is the deposit 

 of the Wisconsin period. In some places in our immediate 



