ACTUAL ELEVATION OF STONE RIDGES BY ICE. 
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sloping river-bank, and (d) the summer level of the stream. The occurrence of 
these supra-riparial ridges of angular blocks “ in situ ” is thus explained. When 
the Dwina is at its maximum height (e), 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 con- 
sequent 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 to 
fifteen or twenty 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 ( b ), as evidences of the highest ice-mark 1 . In Lapland, M. Bohtlingk has 
adduced some extraordinary examples of this sort of glacio-fluviatile action ; for 
he assures us that he there found large granitic boulders, weighing several tons, 
actually entangled and suspended like birds’-nests in the branches of pine-trees at 
heights of thirty or forty feet above the summer level of the streams. 
Elevated Bloch Ridges on the Banks of Lakes . — Until we observed the angular 
and elevated block ridges on the banks of the Dwina, and assured ourselves that 
ice is a “ vera causa” in elevating large stony masses, we had great difficulty in 
explaining the origin of certain ledges, which stand at higher levels on the western 
side of the lake of Onega. The chief of these, which was pointed out to us by 
Colonel Armstrong of Petrozavodsk, occurs on the slopes of the hill Kamenibor, 
and is composed of the hard quartzose sandstone or altered old red sandstone, 
described in the fourth chapter. Lying, as nearly as we could guess by the eye, at 
about 200 feet above the lake, this ledge (6'), as represented in the subjoined section, 
1 Though we were only present in the summer season, and therefore could not witness such pheno- 
mena, we have before us a description of an extraordinary breaking-up of the Dwina at Archangel by 
our kind friend Mr. Whitehead, Her Britannic Majesty’s Consul at that port, to whom we had written on 
the subject, which satisfies us, that if the edge of the river there consisted of jointed thin-bedded lime- 
stone, like that to which we allude, instead of the fine mud and sand, which alone constitute its banks, 
the same results must have ensued lower down the stream. Mr. Whitehead’s account is as follows : — “ I 
wish you could witness such a breaking- up of our river as we had in the spring of 1835. In the course 
of five or six hours the water rose fourteen or fifteen feet, with the ice one compact mass upon it. Calcu- 
late the enormous pressure of such a body of water with the impetus of such a current, and you may, 
perhaps, form some idea of the crash when the ice did give way. It was grand in the extreme, and if 
we could calculate upon such a breaking up this spring, I should say you would be richly repaid by 
coming to see it. I could compare it to nothing but the roaring of artillery. Blocks of ice remained 
for a long time high and dry upon the banks.” 
4 d 2 
