Geology . tly because they are rela-tively rare, and partly because they are often rather striking topo-graphic features. They are often conspicuous, not so much becauseof their height, as because of their abrupt slopes and their even andnarrow crests. They may be ten or several times ten feet high, buttheir crests are generally no more than a few feet wide. They are,for example, often so narrow, and their slopes so steep, that two wagonscould with difficulty pass each other on their tops. The angle of their THE PLEISTOCEXE OR GLACIAL PERIOD 375 slopes is about the angle at which the drift wil
Geology . tly because they are rela-tively rare, and partly because they are often rather striking topo-graphic features. They are often conspicuous, not so much becauseof their height, as because of their abrupt slopes and their even andnarrow crests. They may be ten or several times ten feet high, buttheir crests are generally no more than a few feet wide. They are,for example, often so narrow, and their slopes so steep, that two wagonscould with difficulty pass each other on their tops. The angle of their THE PLEISTOCEXE OR GLACIAL PERIOD 375 slopes is about the angle at which the drift will lie. Where they crossmarshes and swamps, as is sometimes the case, they are most con-spicuous, sometimes resembling railway grades. Eskers no morethan a fraction of a mile in length are more common than longer ones,but eskers scores of miles long are known. Long eskers sometimeswind up and down over low elevations and valleys, showing that thewater which made them must have been under great head, if they. Fig. 505.—An esker 10 miles west of Aurora, 111. (Bastin.) are of strictly subglacial origin. They often lie along the lower slopeof a valley, though distinctly above its bottom. Eskers are likelyto be interrupted at intervals, probably at points where the deposit-ing waters failed of confinement to definite channels, or their channelswere too constricted, or had too high gradient to permit of best-developed eskers in the United States are in Eskers are made up primarily of stratified gravel and sand. Asin kames, the stratification is often much distorted, probably as theresult of ice pressure. Bowlders are often present in them and on 1 Stone, Mono. XXXIV, U. S. Geol. Surv. 370 GEOLOGY. (heir surfaces, showing the presence of the Lee during their bowlders might have been crowded in from the sides, or let down from the ice above. As in kames, the gravel is often not well often end in kames, and where they are interrupted,
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