296 
MR. PRESTWICH ON THE GEOLOGY OE THE DEPOSITS 
In our country the effect of frost on freshly exposed perpendicular surfaces of chalk, 
sandstone, and oolites is very marked. The former especially disintegrates very rapidly. 
I have seen a low cliff of chalk 15 feet high form a talus at its foot, in the course of one 
ordinary winter, 6 feet broad by 5 feet high. The wetter the ground the greater, neces- 
sarily, are the effects of the frost ; so, as we assume in our hypothesis that the excavation 
of C (fig. 18, p. 298) was effected by the removal of successive layers (c 1 -*? 4 ), commencing 
with the one at the base of D, each layer, when first uncovered, having been at or near 
the level occupied by the river, must have been at or near the line of general water-level 
or of springs, and therefore more largely and constantly charged with moisture than 
the same strata on higher ground, and such surfaces consequently presented conditions 
the most favourable for the operation of frost. 
Sir R. Murchison gives some very illustrative instances of what he appropriately terms 
recent “ fluvio-glacial action.” Amongst others, in speaking of the Dwina, about sixty- 
four miles above Archangel, where it flows over a white limestone in horizontal layers, 
he remarks, “ About 30 feet above the summer level of the stream, the terrace on the 
river-side is covered for two or three versts by a band of irregularly piled loose and large 
angular blocks of the same limestone, arranged in a long uniform ledge In other 
words, these materials (all purely local) constitute a broken ridge of stones between the 
road and high-water mark When the Dwina is at its maximum height, the water, 
which then covers the edges of the thin beds of horizontal limestone, penetrates into 
its chinks, and when frozen and expanded, causes considerable disruptions of the rock, 
and the consequent entanglement of stony fragments in the ice. In the spring the 
fresh swollen stream inundates its banks (here very shelving), and upon occasions of 
remarkable floods so expands that in bursting it throws up its icy fragments 15 or 20 
feet above the highest level of the stream. The waters subsiding, these lateral ice-heaps 
melt away and leave upon the bank the rifted and angular blocks as evidence of the 
highest ice-mark Dr. Bigsby also gives a section in illustration of a like case on the 
banks of the Ottawa river f. 
Besides the ordinary eroding power of running water, we have had therefore three 
main causes in operation in those regions at the period under consideration: viz., 1. the 
taking-up of the shingle and boulders along the sides of the rivers by the shore-ice, 
and its transport thereby to points lower down the river ; 2. the action of the ice forming 
on the bottom of the rivers, and lifting, as it rose to the surface, shingle and boulders 
from the river-bed and carrying them also to a distance down the stream ; and 3. the 
rending and disintegration of the rocks by frost. The districts traversed by the rivers 
whose courses have been described are peculiarly favourable for the operation of these 
causes, being formed essentially of sands, chalk, soft sandstones, and fissile limestones ; 
not that the harder rocks do not yield to the influence of the same causes, but that 
the others are more readily and quickly affected. 
The combined operation of these causes is visible in many of the rivers of Russia at 
* The Geology of Russia, &c., pp. 566, 567. t Quart. Journ. Geol. Soc. vol. vii. p. 235. 
