352 
Frank Carney 
both factors being active, approximates unity, we have the com- ! 
bination most favorable to producing valley loops. 
(4) Texture of the Drift. The nature of the debris being accu- 
mulated likewise exercises some control over the development i 
attained by the loop. This control is shown more perhaps in the ' 
form of the loop. The coarser the material being assembled, the 
higher will be the loop. When the drift contains a large percent- 
age of clay, which when moist has a tendency to slump, the loop | 
will be low but broad; when the drainage of the valley is ponded, [ 
this slumping will be more pronounced; but even in the absence jj 
of static waters, which induce a genetic broadening of loops, post- jj 
glacial weathering tends to flatten them if much clay is present. | 
Likewise the condition of its gravel content, whether fine or coarse, | 
if sufficiently abundant, manifests itself in the slope of the loop. ! 
THE OUTLINE OF VALLEY LOOPS. | 
Since this form of drift marks the front of the ice, it is evident 1 
that there are controls to which the shape of the ice-tongue itself is | 
subject, and which in ultimate analysis determine the outline of | 
the loop : 
(i) Obviousl}^ the vjidth of the valley, a feature contingent upon 
stage of development and upon the stratigraphy, will give two I 
types of loops. In a valley of gently sloping side walls, the form 
usually found in areas of fairly homogeneous rock structure, the 
loop formed is more symmetrical. It consists of two divisions, i 
the flood plain segment, and the lateral segments. When the 
valley is wide, the flood plain segment is relatively narrower, a 
condition due to the tendency of the ice-tongue to protrude along 
the axis of the valley. The lateral segments consist each of two 
arcs. The portion higher up the valley wall has a longer radius, 
i. e., slighter curvature, than the portion near the flood plain seg- I 
ment. It is apparent that the arc of the loop flattens as we pass j 
along it in either direction from the axis of the valley. 
But valleys having steep side walls, a condition due either to 
lack of maturity or to less resistant rock in which the valley is 
floored, underlying a more resistant formation, tend to shorten 
the arc of the flood plain segment of the loop; that is, the portion 
of the loop in the bottom of the valley is narrower than in the | 
former case. 
