CRITERIA FOR AQUEO-GLACIAL SEDIMENTATION. 39 
temporary existence of a glacial stream entering the basin of deposition near 
the place where such layers are found. On the retreat of the ice the course of 
this stream would shift and empty into the basin at some other point. 
C. Even in the regular bands of finest clay there are often variations of 
considerable amounts in the thicknesses of the annual layers. These are what 
would naturally be expected in annual deposits. If any season should be 
much warmer than the normal season, the amount of water from the melting 
would be greatly in excess of normal and hence the stream flow would be greatly 
increased. As the strength of a current varies as the 6th power of the velocity, 
and the eroding power varies between the 3rd and 5th power, the greatly 
increased velocity of the stream, due to increased melting, would carry much 
more material into the basin of deposit than usual. As a stream has a greater 
capacity for fine than coarse sediment the seasonal layers of fine material should 
show considerable thickening as a result of an unusually warm season. The 
rock flour and gravel layers mentioned above might result thus. 
An abnormally rainy summer would produce the same result as a warm 
summer. Rain has the power of melting ice with great rapidity. Whether 
or not it would be possible to distinguish between these two causes of increased 
sedimentation, it is not possible, at present, to say. Although abnormal heat 
or rains would produce unusual thickening of the layers, small changes in stream 
flow might also be registered in the deposits. 
D. In some of the seasonal banding in which the layers are composed of 
coarser materials, there may be observed sudden changes from thick to thin 
layers or vice versa. Where such sudden changes come it is more reasonable 
to ascribe the change to shifts of the glacial stream from year to year than to 
changes in the weather. It is observed that there is a marked lack of uniformity 
in interval between the layers of the coarser sediments. Where the texture of 
the material is of sand or fine gravel, uniformity of interval between the bands 
is rare. Currents swift enough to deposit sand or gravel would also be strong 
enough to erode any bed of finer material which had been deposited. In this 
way the fine component would be partially or wholly lost. This erosive action 
combined with the shifting of the streams would complicate the bedding in 
such a way that it would be impossible to recognize seasonal effects. With 
these facts in mind it is not difficult to understand the irregular banding in 
deposits of alternating fine sand and silt, and sand and gravel. (See Plate 3). 
In spite of these highly disturbing factors in the coarser sediments, however, 
regular banding has been observed in alternating beds of conglomerate and slate, 
at Squantum. (See Plate 4). 
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