CENOZOIC TIME — QUATERNARY. 



963 



The nearness of the moraine-line A, or the southern ice-limit, to that of moraine-line 

 B in Pennsylvania may be owing to the fact that the course of each was not dependent 

 on the isotherms, but on the leeward position of the region with reference to the icy 

 heights to the northeastward (page 956). 



2. Deposition and distribution of drift. — With the melting and retreat 

 of the ice-sheet, deposition of the transported material went forward making 

 a covering of till of varying thickness, deposits in some parts of clay and 

 rock-flour over and within the till, and intercalated deposits also of soil, 

 sometimes with remains of forests, as has been already described. Besides, 

 the escaping waters carried away material, fine and coarse, for stratified beds 

 of clay, sand, and gravel. The older till over Illinois and Indiana has usu- 

 ally a depth of about 20 feet. In southeastern Indiana and southwestern 

 Ohio, according to Leverett, it was followed by a covering of soil and then 

 a deposit of clay to a depth of several feet; and as the clay contains, 

 according to an analysis, 2'32 per cent of potash and soda, 16 per cent of it 

 or more is feldspar in grains. The beds of soil and the forest-beds in glacial 

 deposits are mostly contained in those that were made during this retreat. 



1550. 





Upper part of Moraine, Dogtown Commons, Cape Ann. Shaler, 18S9. 



The moraine ridge, which marks the limit of the retreat, consisting chiefly 

 of gravel, stones, and bowlders, was made by the deposition, along the front, 

 of material brought down by the ice-sheet during a long halt. It indicates 

 the transporting power of the ice; and as the moraine in Illinois and Iowa is 

 over 150 miles north of the southern ice-limit, the surface of the ice-sheet may 

 have had a steeper pitch than during the period of maximum ice, so that 

 transportation went on more rapidly, while corrasion and deposition were less 

 effective agencies of rock-wear. The halt had, as usual, its advances and 



