326 THE POPULAR SCIENCE MONTHLY, 



as if, in an enormous denudation of New England, the aggregated 

 material, scoured from its hills and valleys, had been dropped just 

 upon their outskirts in this long detrital barrow or mound. Yet 

 over New England this same deposit is wide-spread ; it lies up and 

 down the valleys, it forms the terraces of its rivers, the shores of its 

 lakes, and, spread over the face of the land, is frequently the immedi- 

 ate soil beneath the feet. This member of .the geological series, 

 exhibiting various phases in its deposition, from the bowlder-clay 

 to the lake-ridges, is widely distributed, indeed is universal over the 

 Northern States, and as far south as 40 north latitude extends its 

 sheets and centres of pebbly and sandy deposit in mounds and 

 ridges, themselves capped with accidental bowlders, and resting upon 

 the furrowed and seamed surfaces of the rock beneath. Sometimes 

 they may be found collected in heaps and walls at the foot of the 

 polished rocks, as if silent and incontrovertible witnesses of their 

 severe and prolonged erosion. 



In Scotland it is the till, a stiff clay interspersed with polished 

 stones, crowding down the valleys and prevalent over the lower 

 slopes, varying in its lithological character with the character of the 

 surrounding rocks. Gravel and sand beds are intercalated with it 

 and superimposed upon it. In England, Ireland, Scandinavia, Swit- 

 zerland, we discover identical strata strata which, while yielding dif- 

 ferent subdivisions, in their entire extent are the same thing, and only 

 varied according to the local force and extent of the wearing agent, 

 the local peculiarities of the country over which it operated, and the 

 effect which submergence beneath the sea had in redistributing and 

 rearranging the beds of detritus already laid down. In the sequel 

 we shall more particularly revert to this drift-material, and indicate 

 the part it has played in the economy of our landscape-changes ; how 

 it constitutes the terraces of our rivers and the successive beaches 

 of our Great Lakes, and how it has choked up the former courses of 

 rivers, forcing them to find new ones by larger and circuitous deflec- 

 tions. Associated with this phenomenon are the appearances known 

 as crushed ledges and roches moutonnees, both of which testify to 

 the exertion of enormous pressure the one of pressure continuous 

 and progressive, the other, perhaps, of percussive and intermittent 

 attacks. 



Crushed ledges designate those plicated, overthrown, or curved 

 exposures where parallel laminse of rocks, as talcose schist, usually 

 vertical, are bent and fractured as if by a maul-like force battering on 

 them from above. The strata are oftentimes tumbled over upon a 

 cliff-side like a row of books, and rest upon heaps of fragments broken 

 away by the strain upon the bottom layers, or crushed off from their 

 exposed surfaces. Moches moutonnees are those rounded and swelling 

 prominences, often seen in a landscape, which, when examined more 

 closely, show themselves to be truncated masses of rock whose asperi- 



