270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
drainage near the edge of the ice-sheet was systematic enough to allow 
the concentration of waste at occasional favorable points along its front. 
In their broader features of form and structure, sand plains resemble 
deltas built under ordinary conditions, by streams entering a body of 
standing water. Like normal deltas, they have a nearly flat, gently 
sloping surface, the “top slope,” and an outer or free border, the “ front 
slope,” which slants more decidedly and is lobate in form ; in composition, 
the material becomes noticeably finer towards the free border ; in struc- 
ture, the sand plains show inclined beds below with horizontal beds of 
coarser material above. Yet while sand plains are delta-like in these 
respects, the peculiar conditions under which they are built give them 
certain very definite characteristics not common to ordinary deltas. 
Instead of being fan-shaped deposits built out from lake-shores by single 
streams, sand plains are more often semi-elliptical in outline, as if built 
out continuously from the ice for some distance along its front by many 
streams. In fact, they are most irregular in plan. 
The headward border of a sand plain, moreover, is not the lake-shore 
border of a common delta, but an “‘ice-contact”’ or “back slope,” which 
marks the position of the ice-front against which the gravels were laid 
down. Inasmuch as the back slope shows exactly where the ice-front 
was when the sand plain was built, it is a great help in determining the 
ice boundary of the ice-front lake at that time. In linear extent the 
back slope is straight or irregular, according as the ice-front was straight 
or irregular. In surface form it may be a simple straight slope of from 
30 to 45 degrees, —the natural slope or ‘‘angle of repose” assumed by 
the gravels when their ice support was removed by melting. It is 
frequently broken, however; by hollows or kettle-holes, where isolated 
blocks of ice were enclosed in the gravel, and occasionally the ice-margin 
of a plain is wholly a belt of knobs and kettles, indicating an irregular 
ice-front where deposition took place among ice-blocks or upon a thin 
irregular ice-margin. In some cases, also, the ice-border is marked by a 
high moraine which rises above the surface of the plain, and is bounded 
on the back by the usual steep ice-contact slope. More typically, how- 
ever, the back slope meets the top slope of the sand plain at a sharp 
angle or shoulder. At the back of the plain the material is coarsest, — 
usually cobbly gravel with boulders, which have reached the ground 
either by tumbling or melting from the ice. 
The top slope is often so flat as to appear like a level plain; but 
usually there is a perceptible slope, 3 to 5 degrees, from the ice-border 
towards the free border. Near the back of the plain, kettle-holes are 
