182 ILLINOIS STATE ACADEMY OF SCIENCE 



ern unit of the quadrangle without great difficulty, but 

 when it encountered the rugged upgrade slopes of the 

 Pottsville unit, the forces which were pushing it ahead 

 seem to have been insufficient to send it completely up 

 the grade. The southward progression of the glacier, 

 therefore, is thought to have ceased more because of a 

 lack of motion of the ice itself rather than to the predom- 

 inance of melting over ice advance. There was probably 

 a geologically brief period when the advance and the 

 melting back of the ice were equal and the margin there- 

 fore maintained a fairly constant position. During that 

 time deposits accumulated at the ice front in thicknesses 

 somewhat in excess of those formed elsewhere in this 

 region, but in this rough topography did not form a 

 prominent moraine. 



The first important event in the general glacial history 

 of the region was the shutting off of the drainage of 

 Craborchard Creek from Big Muddy River. It is not 

 known where the ice first interrupted the drainage of the 

 creek, whether near Big Muddy River or farther up 

 stream, but eventually the result was the same. All the 

 precipitation as well as the water from the melting of the 

 glacial ice was therefore ponded in that part of the val- 

 ley of Craborchard Creek not occupied by ice and its 

 tributaries until it became high enough to cross the di- 

 vide at the headwaters of the creek some 20 miles east 

 of Carbondale near New Dennison. Just how high the 

 water stood in the valley can now only be approximated, 

 but it certainly stood as high as 435 feet above sea level, 

 for that is the present height of the New Dennison col. 

 The maximum depth of the lake was about 100 feet. 

 With the continued advance of the ice, certain northward 

 flowing tributaries of Craborchard Creek were cut off 

 from the main stream and became local lakes. Some of 

 these lakes at times found exit across divides into neigh- 

 boring valleys which still discharged into Craborchard 

 Creek. Two cols which were produced in this fashion, 

 but probably during the early stages of Lake Craborch- 

 ard, occur in Sees. 1 and 2, T.10 S., R.l E., where the 

 waters of Little Grassy and Caney creeks probably 

 crossed the divide into Sugar Creek and thence joined 

 the main drainage. 



