iS94-] 201 [Woodworth. 
gravels and sands were originally deposited. This slojse may also 
originate in till accumulated along the ice-fi-ont. There are 
hummocky slopes less than the angle of repose in which the 
toi)ography is due to the same cause, the difference aiising from 
the mode of deposition upon a gently inclined rather than a 
steep surface of ice. Some hillside kames belong here. This 
slope marks the contact of the ice-sheet or its remnants with 
the detritus deposited aboixt its base. It is then a contact-line^ 
and is to the glacial geologist what contacts and unconformities 
are to the student of the older rocks. The materials on the 
slope are differentiated from those in the mass through their 
arrangement by gravity, and by reason of their subsequent deposi- 
tion on the slope. Professor Chamberlin^ has proposed a system 
of mappuig topographic forms to show relative chronology. The 
moraine terrace slope and its homologues in the heads of glacial 
sand-plains and the slopes of typical eskers may then be colored 
to show (1) contact between ice and glacial drift, (2) the dis- 
tribution of gravity slopes (taluses), or (3) contemporaneous 
glacial terraces. 
These slopes are to be distinguished from those formed by the 
adjustment to gravity of terraces of erosion. Gilbert has given 
the differentiae in his monograph on Lake Bonneville. The 
slopes must also be distinguished from those made in glacial 
deposits by the liquefaction of buried ice and the settling of an 
ovei'lying cover of drift. 
Steep sides are characteristic of typical eskers. Like the 
slojjes of the moraine terrace, they mark the contact with walls 
of ice once existing on either side of the ridge of gravel. The 
typical moraine terrace is exterior to the ice-sheet : the esker is 
interior. Eskers, then, since they extend often nearly unbroken for 
miles into the interior of a now vanished ice-sheet, must bear in 
their form and structure indications of the conditions which there 
prevailed during the period of their formation, and their impor- 
tance as geological monuments is the greater if we find them to 
be of the class formed near or at the base of the ice-sheet. The 
relation of slopes to crest-line elevation will be discussed in the 
following section. 
' A proposed system of chronologic cartography on a physiographic basis. Bull. 
G. S. A.,v. 2, p. 541-544, 1891. 
