
STRUCTURE AND SURFACE FEATURES 421 
lakes may be formed in valleys by the disproportionate 
accumulation of sediment by a river and its tributaries. The 
river, by carrying down large quantitics of material, may 
gradually raise the surface of its bed above that of its 
affluents, in the lower reaches of which barrier-lakes will 
thus be formed. Or the tributary streams may throw more 
detritus into the main valley than the river occupying the 
latter can carry away. Barriers are thus produced, and large 
valley-lakes appear above the obstructions, of which some of 
the lakes in Upper Engadine (Silser See, Silvaplana See) are 
examples. 
(¢) A©olian Basins.—These are more interesting than 
important, and are naturally confined to relatively dry 
regions. Some owe their origin to the erosive action of wind, 
while others are constructional—that is to say, they are 
hollows lying amongst wind-blown accumulations. 
(7) Rock-fall Basins.—These are caused by -landslips, 
etc., obstructing the drainage, and are usually of little 
importance. As we should expect, they are of frequent 
occurrence in regions of recent mountain-uplift, where the 
geological structures are weak and liable to collapse. 
(g) Glacial Basins.—The basins included under this 
head are of various origin—some being true hollows of 
erosion, others constructional, z.e. due to the unequal heaping- 
up of detritus, while many are partly one and partly the 
other. Hence, some lakes of glacial origin occupy true rock- 
basins, and others are essentially barrier-basins. All the 
large tectonic basins, in so far as they are due to crustal 
deformation, might be described as rock-basins, while certain 
voleanic basins would likewise come under the same category. 
But in these cases the depressions are obviously related 
to geological structures—geosynclines, dislocations, crateral 
hollows. Rock-basins of glacial origin differ from all others 
in the fact that they are totally independent of geological 
structure and the character of the rocks themselves. They are 
met with alike in igneous, metamorphic, and derivative rocks 
—whether these be relatively “hard” or “soft”—and they 
occur indifferently in regions of horizontal, gently inclined, 
highly flexured, and contorted strata. As might have been 
expected, both rock-basins and barrier-basins of glacial origin 

