Page:Popular Science Monthly Volume 12.djvu/340

This page has been validated.

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 immediate 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, Switzerland, we discover identical strata—strata which, while yielding different 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 deflections. Associated with this phenomenon are the appearances known as crushed ledges and roches moutonnées, 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 laminæ 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. Roches moutonnées 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-