8 BULLETIN 141, U. S. DEPARTMENT OP AGRICULTURE. 



at the south maintained the first glacial-lake stages at altitudes as 

 high as 1,100 feet in the Finger Lake region of central New York, 

 while the later more extended glacial lakes in western New York 

 occupied areas lying below 800 feet in elevation and thence declining 

 to an altitude of 246 feet, the present elevation of the surface of Lake 

 Ontario. At all of these elevations continuous or interrupted areas 

 of lake sediments were deposited. 



The materials which were reworked and redeposited as glacial- 

 lake sediments were everywhere of diverse origin. They were de- 

 rived immediately from the heterogeneous mass of stone, gravel, sand, 

 silt, and clay which formed the glacial till. This mass of earthy 

 matter existed either upon or in the ice mass and was set free by 

 melting or it had previously been laid down by the ice in the form 

 of moraines and other deposits. In some localities it is possible that 

 the glacial lake waters also derived some material from underlying 

 consolidated rocks where these were not covered by glacial deposits. 

 Such occurrences were of very small extent. 



In general, the largest amounts of material were contributed either 

 by the glacial streams which flowed directly from the ice, by streams 

 which flowed into the glacial lake basins from the uncovered but 

 previously glaciated uplands, or through the direct action of waves 

 and currents of the glacial lakes upon the till which formed the 

 boundaries or the floor of the glacial lake basins. 



The streams which were formed directly from the melting of the 

 ice carried glacial materials of all sizes, which were sorted and de- 

 posited either in the form of long, low ridges chiefly consisting of 

 gravel and sand, which are known as eskers, or as broad, low out- 

 wash plains usually sandy in their general character. In both cases 

 the finer sediments were carried to positions more remote from the 

 ice front and were deposited in the deeper and quieter waters of the 

 glacial lakes. 



Similarly the streams which flowed into the glacial-lake basins 

 from the deglaciated uplands brought large amounts of material 

 and this was partially or completely assorted to be deposited in the 

 form of stream deltas near the outer margin of the lake areas. Fre- 

 quently the coarser material was dropped in the form of low alluvial 

 fans where the stream waters entered the lake. The finer materials 

 from these sources were also carried to greater distances and de- 

 posited with the finer sediments derived directly from the glacier. 

 There was thus a mingling, in the majority of instances, of upland 

 glacial till, of local country rock material, and of materials con- 

 tributed directly through the melting of the glacial ice, all deposited 

 to form the different grades of glacial lake sediments. 



Along the landward border of each of the larger lakes wave action 

 played a considerable part in eroding both the glacial till and in some 



