CHAPTER IV.

SIGNS OF PAST GLACIATION.

2. Exten

THE facts from which we draw the inference that vast areas of the earth's surface which are now free from glaciers were, at a comparatively recent time, covered with them, are fourfold, and are everywhere open to inspection. These facts are: 1. Scratches upon the rocks. sive unstratified deposits of clay and sand intermingled with scratched stones and loose fragments of rock. 3. Transported boulders left in such positions and of such size as to preclude the sufficiency of water-carriage to account for them. 4. Extensive gravel terraces bordering the valleys which emerge from the glaciated areas. will consider these in their order:

1. The scratches upon the rocks.

We

Almost anywhere in the region designated as having been covered with ice during the Glacial period, the surface of the rocks when freshly uncovered will be found to be peculiarly marked by grooves and scratches more or less fine, and such as could not be produced by the action of water. But, when we consider the nature of a glacier, these marks seem to be just what would be produced by the pushing or dragging along of boulders, pebbles, gravel, and particles of sand underneath a moving mass of ice.

Running water does indeed move gravel, pebbles, and boulders along with the current, but these objects are not held by it in a firm grasp, such as is required to make a groove or scratch in the rock. If, also, there are inequali

ties in the compactness or hardness of the rock, the natural action of running water is to hollow out the soft parts, and leave the harder parts projecting. But, in the phenomena which we are attributing to glacial action, there has been a movement which has steadily planed

FIG. 19.-Bed-rock scored with glacial marks, near Amherst, Ohio. (From a photograph by Chamberlin.)

down the surface of the underlying rock; polishing it, indeed, but also grooving it and scratching it in a manner which could be accomplished only by firmly held graving-tools.

This polishing and scratching can indeed be produced

by various agencies; as, for example, by the forces which fracture the earth's crust, and shove one portion past another, producing what is called a slicken-side. Or, again, avalanches or land-slides might be competent to produce the results over limited and peculiarly situated areas. Icebergs, also, and shore ice which is moved backwards and forwards by the waves, would produce a certain amount of such grooving and scratching. But the phenomena to which we refer are so extensive, and occur in such a variety of situations, that the movement of glacial ice is alone sufficient to afford a satisfactory explanation. Moreover, in Alaska, Greenland, Norway, and Switzerland, and wherever else there are living glaciers, it is possible to follow up these grooved and striated surfaces till they disappear underneath the existing glaciers which are now producing the phenomena. Thus by its tracks we can, as it were, follow this monster to its lair with as great certainty as we could any animal with whose footprints we had become familiar.

2. The till, or boulder-clay.

A second sign of the former existence of glaciers over any area consists of an unstratified deposit of earthy material, of greater or less depth, in which scratched pebbles and fragments of rock occur without any definite arrangement.

Moving water is a most perfect sieve. During floods, a river shoves along over its bed gravel and pebbles of considerable size, whereas in time of low water the current may be so gentle as to transport nothing but fine sand, and the clay will be carried still farther onwards, to settle in the still water and form a delta about the river's mouth. The transporting capacity of running water is in inverse ratio to the sixth power of its velocity. Other things being equal, if the velocity be doubled, the size of the grains of sand or gravel which it transports is in

creased sixty-four fold.* So frequent are the changes in the velocity of running water, that the stratification of its deposits is almost necessary and universal. If large

fragments of rocks or boulders are found embedded in stratified clay, it is pretty surely a sign that they have been carried to their position by floating ice. A small mountain stream with great velocity may move a goodsized boulder, while the Amazon, with its mighty but slow-moving current, would pass by it forever without

* Le Conte's Geology, p. 19.

stirring it from its position. But the vast area which is marked in our map as having been covered with ice during the Glacial period is characterised by deep and exten

FIG. 21.-Typical section of till in Seattle, Washington State, about two hundred feet above Puget Sound. This is on the height between the sound and Lake Washington.