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 direct 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 increased sixty-four fold.[AS] 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 good-sized boulder, while the Amazon, with its mighty but slow-moving current, would pass by it forever without 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 extensive deposits of loose material devoid of stratification, and composed of soil and rock gathered in considerable part from other localities, and mixed in an indiscriminate mass with material which has originated in the disintegration of the underlying local strata. [Footnote AS: Le Conte's Geology, p. 19.]