=Eskers.=—Intra-morainal features, or those developed beneath the glacier but relatively near its margin, include the “serpentine kame”, _esker_, or, as it is called in Scandinavia, the _os_ (plural _osar_) (Fig. 340). These diminutive ridges have a width seldom exceeding a few rods, and a height a few tens of feet at most, but with slightly sinuous undulations they may be followed for tens or even hundreds of miles in the general direction of the local ice movement (Fig. 341). They are composed of poorly stratified, thick-bedded sands, gravels, and “worked over” materials, and are believed to have been formed by subglacial rivers which flowed in tunnels beneath the ice. Inasmuch as the deposits were piled against the ice walls, the beds were disturbed at the sides when these walls disappeared, and the stratification, which was somewhat arched in the beginning, has been altered by sliding at both margins. As already stated, eskers have not a general distribution within the glaciated area, but are often found in great numbers at specially favored localities. Formed as they are beneath the ice, it is believed that many have their materials redistributed so soon as uncovered at the glacier margin, because of the vigorous drainage there. They are thus to be found only at those favored localities where for some reason border drainage is less active, or where the ice ended in a body of water. =Drumlins.=—A peculiar type of small hill likewise found behind the marginal moraine in certain favored districts has the form of an inverted boat or canoe, the long axis of which is parallel to the direction of ice movement, as is that of the esker (Fig. 342). Unlike the esker, this type of hill is composed of till, and from being found in Ireland it is called a _drumlin_, the Irish word meaning a little hill (Fig. 343). Drumlins are usually found in groups more or less radial and not far behind the outermost moraine, to which their radiating axes are perpendicular. The manner of their formation is involved in some uncertainty, but it is clear that they have been formed beneath the margin of the glacier, and have been given their shape by the last glacier which occupied the district. The mutual relationships of nearly all the molded features resulting from continental glaciation may be read from Fig. 344. [Illustration: FIG. 343.—View of a drumlin, showing an opening in the till. Near Boston, Massachusetts (after Shaler and Davis).]