A type of glacial deposit of particular interest is the "drumlin" which is, in reality, only a special form of ground moraine material (commonly till), and, therefore, essentially unstratified. Typical drumlins are low, rounded mounds of till with roughly elliptical bases and steeper fronts facing the direction from which the ice flowed. Their long axes are always parallel to the direction of ice movement. In height they commonly range from 50 to 200 feet. Their mode of origin is not yet definitely known, but they form near the margins of broad lobes of ice either by erosion of earlier glacial deposits, or by accumulation beneath the ice under peculiarly favorable conditions, as perhaps in the longitudinal crevasses. One of the finest and most extensive exhibitions of drumlins in the world is in western New York between Syracuse and Rochester. Thousands of drumlins there rise above the general level of the Ontario plain, the New York Central Railroad passing through the very midst of them. Drumlins are also abundant in eastern Wisconsin. Another type of glacial deposit in the form of low hills is the "kame" which, unlike the drumlin, always consists of more or less stratified material. Kames are seldom over 200 feet high, and they are of various shapes. In many cases they form irregular groups of hills, and in other cases fairly well defined kame ridges. Kames form as deposits from débris-laden streams emerging from the margins of glaciers, the water sometimes rising as great fountains because of the pressure. Such deposits are now actually in process of formation along the edge of the great Malaspina Glacier of Alaska. Kames are commonly associated with terminal and recessional moraines. "Eskers" are similar except that they are long winding low ridges of stratified material deposited by débris-laden streams, probably in longitudinal fissures in the ice near its margin. (See Plate 20.) Glacial bowlders, or "erratics" are blocks of rock or bowlders left strewn over the country during the melting of the ice. They vary in size from small pebbles to those of many tons of weight, and most of them were derived from ledges of relatively hard, resistant rocks. (See Plate 20.) Erratics have very commonly been carried a few miles from their parent ledges, while more rarely they have traveled even hundreds of miles. They are extremely abundant in New York and New England, many occurring even high up on the mountains. In some cases erratics of ten or more tons' weight have been left in such remarkably balanced positions on bedrock that a child can cause one of them to swing back and forth slightly. Such a bowlder is literally a "rocking stone." In the Adirondack Mountains the writer recently observed a rounded erratic of very hard rock fourteen feet in diameter resting in a very remarkably balanced position on top of another large round glacial bowlder. CHAPTER VI THE ACTION OF WIND