276 
ME. PEESTWICH ON THE GEOLOGY OE THE DEPOSITS 
became deepened to c, the gravels d, though raised above the river-channel, would still 
be subject to be covered on the occasion of the flood-waters rising to n, and the Loess 
would accumulate at b', covering the gravel d and sloping up against the side of the hill 
to the level of the line n. So again when further deepened to d, the Loess would be 
deposited at b" so long as the flood-waters rose to m, or to any point between c and m. 
Pig. 17 . — Diagram representing one side of a valley with a series of gravel and Loess beds. 
The Loess occasionally contains thin seams of gravel, derived sometimes from the 
underlying gravels d and c, but more frequently from the adjacent land-surface. The 
extreme sharpness, as a rule, of the debris from the latter, leads me to believe that the 
transport of such angular fragments, and the common irregularity of their bedding, arose 
from drifting shore-ice. The usually small size of the fragments and the general absence 
of subangular gravel is also compatible only with the thin ice that might be formed at 
night during the spring floods, and away from the shingle-strewed river-channel. 
The valley-gravels by themselves give no direct evidence of the extent of the floods. 
They merely show, in the quantity of debris, the coarse shingle, and the worn blocks, the 
results of torrential action. If, however, we admit the flood-water origin of the Loess, it 
necessarily follows that, as we find this deposit on ground 50 (if not 100) feet above 
the highest beds of the valley-gravels (which fix approximately the position of the main 
channels of the old rivers), it gives a measure of the floods of that period, and shows them 
to have exceeded even those of arctic rivers at the present day, for the waters of these 
rivers rarely rise more than 40 to 50 feet above their low summer level. This fact fur- 
nishes, therefore, strong corroborative evidence of the scouring and erosive energy of 
these old rivers, and tends to strengthen the opinion, before expressed, of their power to 
excavate, when taken in conjunction with the other agencies before described, the large 
valleys through which the rivers now flow in such dwarfed volumes. I would observe 
that in each valley the height to which the Loess rises above the high-level valley- 
gravels is proportionate to the length of the valley and the area drained, showing 
therefore its dependence upon them, and that it is not referable to any uniform level or 
to any general cause. 
Another character common to the Loess, and pointing, I conceive, to the same flood- 
water origin, is the presence of numerous small worm-like cavities penetrating the mass. 
These appear to me to be likely to have arisen in most instances from the presence of 
the vegetable and animal matter with which flood-waters are always more or less charged, 
and which, deposited with the mud, decomposed and gave rise to gases that, as they 
