CONTAINING FLINT IMPLEMENTS, AND ON THE LOESS. 
287 
when they became covered np with the flood-silt, which, extending also over the 
adjacent land and shores, was there deposited directly upon the rocky substratum. As 
the channel became deeper, and the tributary valleys partook of the same erosion, they, 
being out of the main river-current, tended especially to receive thick deposits of the 
flood-silt (Loess), while the higher grounds were left permanently dry. 
Rivers subject to periodical floods are extremely variable in their course and direction, 
flowing first on one side and then on the other side of the valley, shifting the shoals and 
gravel banks, and distributing them in a very irregular manner. Consequently it is by 
no means necessary to suppose that a bed of gravel like that at Oakley and Abbots 
Thorpe extended across the valley of the Waveney, or that the gravel bed of St. Acheul 
extended the whole width of the valley of the Somme, but we may rather infer that 
local conditions led to the great accumulations of gravel at certain spots, especially on 
the sides, whilst others would be left more or less bare. The subsequent denudation 
may therefore have been comparatively slight, and the present outliers of high-level gravels 
may yet represent a not inconsiderable portion of the alluvium of the old rivers. That 
these rivers had at times a torrential character, is evident from the nature of their 
transporting power, as indicated by the prevalence and coarseness of the gravels, by the 
absence of mud-sediment, and by the rough and irregular lines of bedding. But not 
only have we these exhibitions of the power of the old rivers ; it is further evident, 
from the presence in the terrace-gravels of large blocks, often but little worn and 
transported from considerable distances, together with much sharp and angular smaller 
debris, that there was some other power in operation besides the ordinary transporting 
power of water, great though that be. For the blocks in the one case would have shown 
an amount of wear in proportion to the length of transport, and the smaller debris 
would have been separated from the larger ; whereas the blocks are always more or less 
angular, they are scattered indiscriminately through the gravel, are often associated with 
the most delicate and fragile shells, and with bones of Mammalia but little or not at all 
worn. The only cause adequate to produce these results is, I conceive, the action of 
river-ice, whereby these blocks and a portion of the debris were carried down and 
deposited along the river-channel, more especially in those parts where the currents 
may have been checked either by a widening of the river or by the influx of a tributary 
stream. The recent phenomena, with reference to the transport of blocks by ice on the 
St. Lawrence at its breaking up in the spring, have been so well proved by Captain 
Bayfield* and Sir W. Logan f, and illustrated by Sir Charles Lyell and other geolo- 
gists, that it is unnecessary to enlarge upon them here. I may, however, mention that 
more lately in sinking the caissons for the Victoria Bridge at Montreal, the bed of the 
river, through its width of about two miles, was found to be strewed with large rock 
boulders. 
The remarkable contortions in the clay cliffs of Norfolk have been attributed by 
* Proe. Geol. Soc., vol. ii. p. 223 (1836). 
t Ibid. vol. iii. p. 766, and Canadian Journal. 
2 R 
MDCCCLXIV. 
