CONDITIOIT OP ROCK IN PLACE. 7 



glacial sediments, as compared with the amounts of sand and clay, is caused 

 by the nature of the rocks. 



CONDITIOlSr OF ROCK IN PLACE. 



In Maine, as in a large part of eastern North America, the solid rock 

 has been so planed and scratched by the great ice-sheet that only here and 

 there is there to be found any residue of the preglacially weathered sur- 

 face. The state of preservation of the glacial scratches varies greatly. In 

 Brownville, Munson, and all the roofing-slate region, the scratches are 

 wonderfully well preserved. On broad, level tops of hills, where the 

 wet surface precluded any suspicion that the till had been eroded, I have 

 repeatedly found areas of bare rock several rods in diameter upon which 

 minute scratches, such as might be made by the finest needle jDoint, are 

 still sharply defiiied, and the situation of the rock shows that they must 

 have been exposed to the weather ever since the melting of the ice. But, 

 though the durable Maine roofing slate has preserved almost unchanged 

 the record that was engraved upon it by the drift agencies, it is far other- 

 wise with most of the other rocks. On most of the exposed ledges the 

 glacial scratches have either disappeared or are gradually vanishing because 

 of the weathering of the surface. Over large areas it is already impossible 

 to ascertain the direction of the glacial movement except approximately by 

 the forms of the "roches moutonndes" or, better, by digging away the 

 overlying earth, when the scratches on the subjacent rock will usually be 

 found perfectly preserved. Already some of the ledges are split and 

 weathered to a depth of several inches, and occasionally to a depth of sev- 

 eral feet. All this indicates the condition of the rock before the coming of 

 the ice-sheet. During the unnumbered ages of Mesozoic and Tertiary 

 time all the State was above the sea, and subaerial weathering and erosion 

 had done their long work upon the surfaces of upheaval. The hills and 

 valleys were in nearly their present forms, but the surface was weathered 

 and shattered often to the depth of 50 or even 100 feet. Over most of the 

 State the great glacier removed the weathered rock and planed the surface, 

 but here and there the planing did not reach the bottom of depressions of 

 weathering. 



The weathering of exposed ledges and bowlders has been greatly aided 

 by forest fires and hj the burning of brush in clearing the land. 



