GLACIAL DEPOSITS 599 



example is shown by the fossiHferous layers of Sankaty Head on 

 Nantucket, which, however, lie really within the kame-moraine 

 area rather than in the present apron plain. It is probable, however, 

 that the fossiliferous beds themselves belong to an earlier apron 

 plain, for it is known that the Wisconsin ice sheet advanced over this 

 deposit and built its moraine and apron plain further south. (Wil- 

 son-6o:7j.) 



3. The eskcr. (Swedish os, pi. osars.) This term is applied 

 to long, narrow ridges of sand and gravel with steeply sloping sides 

 and often a sinuous outline. The summit of the esker is generally 

 a narrow, flat space between the sloping sides, the cross-section be- 

 ing very nearly a triangular one. The eskers of the last glacial 

 period run in general parallel with the direction of the ice move- 

 ment and while occurring largely perhaps in the north and south 

 valleys, along which the main drainage lines from the ice discharged, 

 they are not confined to these, but often pass indifferently over hills 

 and through valleys. In height they rarely exceed 100 feet, but in 

 length they may be followed for tens or in rare cases even hun- 

 dreds of miles. Their best development in this country is in New 

 England, wdiile Scandinavia and Finland furnish some of the best 

 illustrations in the Old World. 



Eskers may be formed in various w^ays, such as by filling of 

 gorges in the ice, by accumulation of debris in englacial tunnels and 

 by aggradation of their bed by sub-glacial streams. (Grabau-23.) 

 This last is the most characteristic method of formation. Streams 

 originating under the ice are enclosed in a tunnel and, being under 

 an enormous hydrostatic pressure, will fill this tunnel after the 

 manner of the water in a city main. It may thus be forced up- 

 ward and will discharge at a level much above that of its upper 

 courses, if it is located in a valley sloping against the ice mass, as is 

 so often the case. 



Flowing uphill, such a stream will tend to erode the roof of 

 its tunnel, and simultaneously to aggrade its bottom in the higher 

 (upstream) parts. As the stream rises in the ice by roof erosion, 

 the floor is more and more aggraded until this filling all along the 

 line has reached the level of the outlet, or risen slightly above it. 

 Melting removes the supporting walls on either side of this aggraded 

 river-bed, when the sides will slump, giving the characteristic lateral 

 slopes and the triangular cross-section to the esker. At points 

 where the original deposit is narrow, so much material will be in- 

 volved in the slumping to give the proper angle of repose that the 

 surface of the esker will be actually lowered. Thus irregularities in 

 height in the esker are readily accounted for. (Woodworth-6r.) 



