CHAPTER IX.

ROCKS OF THE QUATERNARY PERIOD.

281. THE Quaternary series embraces the heterogeneous masses of the Drift and all subsequent deposits up to the present period. English geologists generally discard this division, and extend the Tertiary over this series.

THE DRIFT PERIOD.

282. The Drift includes accumulations of sand, gravel, clay, pebbles and boulders, or erratic blocks; the boulders are fragments of the hard crystalline rocks, usually water-worn, rounded, and scratched, or grooved. The term Diluvium has been applied to this deposit; drift indicates that the materials have been impelled by currents.

283. The drift is not universally diffused, but appears to be confined within 40° or 50° from the poles. In Asia it is rarely found lower than 60° north latitude; in Europe it descends to the southern parts of Poland and Prussia; and in North America it is found as low as 40°, and some of the north and south valleys extend it a little farther southward. Its southern limit in the United States is a line drawn from Long Island through central Pennsylvania to the Ohio, with occasional outliers in the valleys of the Delaware, Susquehanna and Mississippi. It is found also in the southern portions of South America, but not within the tropics. It occurs at elevations above the present sea

level, varying from three hundred to five thousand feet. The general direction of the drift in North America has been from north to south, and occasionally southeast; while in Europe it appears to have been dispersed in various directions from the Scandinavian Mountains. Boulders have been transported in some instances hundreds of miles; the largest of them have, however, usually been deposited within a few miles of the ledges from which they were torn. They diminish in size and number as the distance from their original position increases. Boulders of considerable size are frequently found in Northern Ohio, but are few and small in the central and southern parts of the State. They can be identified with the rocks of the parent ledges by their general constitution and by particular minerals contained in them.

284. Boulders vary greatly in size; some of them weigh thousands of tons. A conglomerate boulder at Fall River, President Hitchcock states, weighs ten million eight hundred thousand pounds; a granite boulder near Neufchatel weighs three million eight hundred and fifty thousand pounds; and the boulder from which the pedestal of the statue of Peter the Great was hewn, weighed one thousand five hundred tons. Some boulders are so poised upon hard surfaces, as to oscillate by the application of a moderate force to them, and are then called rocking stones; others are firmly posited in such a manner as to have given rise to the conjecture that they are artificial structures. An example of this occurs in the State of New York, and is represented in Fig. 154. A boulder of felspathic granite weighing about fifty tons, rests at the height of three or four feet above the ground, upon limestone pillars. They are sometimes poised on the summits of mountains; others

have been carried over mountain ridges, though they are usually found accumulated in larger numbers on the north sides of mountains.

285. The transport of the drift has produced very conspicuous effects upon the surface of the earth, scratching, grooving, and polishing the rocks. The scratches and grooves are parallel, resembling those produced by glaciers described in § 32. They vary from a fraction of an inch to more than a foot in depth and width. Sometimes two or more sets of striæ cross each other at a small angle. These striæ have been found on the White Mountains five thousand feet above the ocean level. Prof. Locke gives an example of these grooves on the limestone near Dayton, Ohio. "The quarry has been stripped of soil more or less over ten acres, and the upper layer of stone is in most

places completely ground down to a plane, as perfectly as it could have been done by a stone cutter, by rubbing one slab on another with sand between them. In many places, in addition to the planishing, grooves and scratches in parallel straight lines, evidently formed by the progress of some heavy mass, propelled by a regular and uniform motion, are distinctly visible. The grooves, are, in width, from lines scarcely visible, to those three-fourths of an inch wide, and from one-fortieth to one-eighth of an inch deep, traversing the rock in a direction south 26° east, in lines exactly straight and parallel."

It is probable that a very large portion of the earth's surface was affected in this way, as far as the drift extended, since the removal of clay and other substances which cover the rocks discovers the striæ; rocks that suffer disintegration by atmospheric agency, as limestones, which are partially dissolved by water holding carbonic acid in solution, do not retain their grooves. The direction of the striæ coincides with that of the dispersed drift, and is often modified by the features of the surface; as when it is diverted from the general direction into a valley. Some sets of striæ appear to have been nearly obliterated by others passing over them.

286. The northern and northwestern sides of the ledges of rocks are more worn by the drift agency, and the hills are elongated in this direction, corresponding in appearance with those denominated by European writers roches moutonnees. President Hitchcock has observed very numerous angular fragments of rocks ranged in long narrow lines, extending from the ledges in the same direction with the drift, and overlying that deposit, which he denominates

* Ohio Geological Survey, p. 230.

streams of stones.

The same geologist adduces as instances illustrative of the prodigious violence of the drift agency, the fracture and overturning of perpendicular strata of slate rocks near the summits of hills. As the materials of the drift are generally supposed to have been transported by the agency of ice, these deposits are called glacial beds; they have very few, if any fossils.

287. Overlying the boulder formation occur beds of blue and yellow clay, sand and marl; these are most abundant in lakes, ponds, and river valleys. They sometimes appear to be caused by a new arrangement or assorting of the materials of the drift, producing an inter-stratification of sands, clays, and gravel, and are called altered drift. This appears to have been accomplished beneath the ocean which prevailed over the drift region.

288. The origin of marine terraces has been assigned to the agency of the ocean exerted at this period. Fig. 155 presents a view in the valley of Glen Roy in Scotland, in which two parallel shelves or terraces, level and contin