266 GLACIAL GEAVELS OF MAINE. 



eons had been going on between the elements and the hving rock was 

 testified to by the towers and buttresses with which the rock in vain 

 strengthened its scarps of erosion. While the outlines of the hills were not 

 so beautifully curved as at present, the drainage basins and the relative 

 height of hill and valley were probably about the same. 



It is uncertain to what depth the rock had become weatliered in 

 preglacial time. Over the driftless area of Wisconsin the residual earth 

 has been found by Chamberlin and Salisbury to have a thickness of 4 

 feet. In a region of granitic rocks the residual earth represents but a small 

 part of the rock which has become shattered and more or less disintegrated. 

 In the Blue Ridge of Vii-ginia I have repeatedly seen railroad cuts where 

 the Archean schists Avere weathered and fractured to a depth of 20 to 30 

 feet. In Maine the roofing slates weather with such extreme slowness that 

 the preglacial soil may have been on the average only a few inches thick, 

 and the weakened rock only a few inches more. The sedimentary sand- 

 stones, etc., may have had onl}^ about the same depth as in the driftless 

 area of Wisconsin, but the crystalline and schistose rocks must have been 

 weathered to a much greater depth. Many of the feldspathic rocks have 

 become weathered to a depth of several inches to several feet in postglacial 

 time, and this indicates a deep preglacial sheet or surface layer of soil, 

 subsoil, and bowlders of decomposition. Obviously the actual depth 

 attained depends on the ratio between weathering and transportation. The 

 till is much more abundant in the regions of schistose rocks than in those 

 of slates and sandstones. This of itself is a proof of a greater depth of 

 weakened rock, and in the granitic regions there was a still greater depth. 

 Judging by the cliffs on the south side of Russell Mountain, elsewhere 

 described, I do not think it an extravagant estimate that the rock in pre- 

 glacial time had there become fractured into blocks removable by the ice 

 to a depth of 50 feet. This was in granite, and not a ver}^ easily weathering 

 variety. The depth of rock which had become fractured and more or less 

 weatliered in preglacial time may be estimated at from a few inches up to 

 perhaps 50 feet. 



Since the land had been for a long time above the sea, the larger 

 valleys would have attained a base-level of erosion. Lakes oocupying rock 

 basins, if there had been any, would have been silted up or have been 

 drained by the cutting down of theii- inclosing barriers, or in case of shallow 



