128 G. K GILBERT — GLACIAL SCULPTURE IN WESTERN NEW YORK 



as to produce striie gradually curving from west to south, and the pecu- 

 liarities of drainage under consideration were seen to fall into this general 

 scheme of trends. 



Only a few days of close observation were necessary to show that the 

 drift-ridge hypothesis was inadequate, and it was finally abandoned for 

 all the Medina plain except a tract in Monroe county near the Genesee 

 river. The principal facts leading to its abandonment are connected 

 with the depth of the drift. If the creeks were guided l)y ridges of drift 

 the general depth of drift should be greater in the interstream tracts 

 than under the stream valleys, but the reverse of this obtains through 

 nearly the whole district of peculiar drainage. Along the creeks running 

 ning northeast the drift mantle is relatively thick ; between them it is 

 relatively thin. 



s.eo'E. N.eo'uv., 



Figure 7. — Diayrammatic Section of glacial Furrow and Ridges. 



sh, shale of the Medina formation, dd, drift. DD, region of sub-lacustrine degradation. A, re- 

 gion of sub-lacustrine aggradation. C, creek valley. * The dotted line shows original prpfile of 

 glacial drift. 



The greater part of the surface was flooded by the glacial lake Iroquois, 

 and that lake had an important influence on the topograph}', not merel}^ 

 silting up hollows with its sediments, but washing till from the hills and 

 ridges and reducing the whole surface to a gently undulating plain. This 

 plain is so even that the faint ridges by which the drainage is actuall}' 

 controlled are hardly discernible by the e3'^e. Where the drift was eroded 

 by the agitation of the lake water only its finer material was removed, 

 the boulders being left, and the regions of greatest lacustrine degradation 

 are represented at the present time by tracts of stony land, whose un- 

 fortunate owners, after utilizing the boulders as far as possible in the 

 making of fences, construct great cairns from the surplus without seeming 

 to exhaust the supply. This accumulation of residuary boulders is dis- 

 tinguished from bouldery till by the fact that it is only superficial. At 

 a depth of 2 or 3 feet the evidence of concentration ceases, and the till 

 below is but moderately stony. In a few localities the layer of boulders 

 rests directly on the red shale, and the remnant of finer drift is too small 

 for profitable culture. In belts of lacustrine aggradation, on the other 

 hand, the soil is stoneless clay or sand, or sometimes gravel, and the drift 

 13 deeper, often so deep that the farmer's well does not reach its base. 



