Fig. 340.—View looking along an esker in southern Maine (after Stone).
Pitted plains and interlobate moraines.—Where glacial outwash is concentrated within a long and narrow reëntrant, separating glacial lobes, strips of high plain are sometimes built up which overtop the other glacial deposits of the district. The sand and gravel which compose such plains have a surface which is pitted by numerous deep and more or less circular lakes, so that the term “pitted plain” has been applied to them. The surface of such a plain steadily rises toward its highest point in the angle between the ice lobes. Though consisting almost entirely of assorted materials, and built up largely without the ice margins, such gently sloping pitted platforms are described as interlobate moraines. Upon a topographic map the course of such an interlobate moraine may often be followed by the belts of small pit lakes (see [Fig. 336]).
Fig. 341.—Outline map showing the eskers of Finland trending southeasterly toward the festooned moraines at the margin of the ice. The characteristic lakes of a glaciated region appear behind the moraines (after J. J. Sederholm).
Fig. 342.—Small sketch maps showing the relationships in size, proportions, and orientation of drumlins and eskers in southern Wisconsin. The eskers are in solid black (after Alden).
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.