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THE GIACIAL PERIOD IN NORTH BRITAIN. 331
comparatively sheltered places. The material of which these ridges
are usually composed is of a loose incoherent nature

and the present
slope of their sides is often as steep as what the sand and pebbles can
He at. Now the angle of repose of such stuff in strongly agitated
water is much less than it is in air. Long narrow mounds sloping
steeply on both sides could not have preserved this form when lashed
by the waves and breakers

and I therefore maintain that when the
movement of emergence brought them near the surface of the water
they would have been levelled to the angle of repose which gravel
beaches usually exhibit.
A large iceberg running aground amongst gravel might cause
mounds like the Kaims, but this could take place only in water of
considerable depth. Heavy packs of ice driven forcibly ashore might
perhaps also give rise to similar ridges, although scarcely, I should
think, of such great dimensions. But unless the mounds were formed
high up on the beach beyond the extreme limit of the tide they would
be levelled again by the return of the waves. If formed in this way
they should also contain some littoral shells, and their curves ought,
as a rule, to present the convex side to the land, and not to the sea.
The sea cutting into the face of a bank may no doubt cause a steep
slope on one side, but not a narrow crooked ridge, steep on both sides
like a railway embankment.
The Swedish Osar* and the American Horsebacks seem to resemble
the Kaims and Eskers in their linear character, gravelly composition,
steep sides, and want of fossils. Sir Charles Lyell, in his account
of the Swedish Osar, says he met with only one instance of shells
occurring in them, viz. in the top of a ridge near the Castle of
Upsala

but as the species he mentions consist of edible mollusca
(mussels, cockles, and periwinkles), there may be a doubt as to
whether their presence in this very exceptional instance has not been
due to some accidental cause. Agassiz, in his description of the
glacial phenomena of Maine, says the horsebacks " are unquestionably
of a moraine nature, and yet they are not moraines in the ordinary
sense of the term." Some of these horsebacks run from north to
south

but occasionally they trend from east to west. " This," he
says, " is the case where a morainic accumulation of loose materials
may have been pushed forward along the margin, in front of an ex-
tensive sheet of ice moving southward, and then left unchanged by
the subsequent retreat northward of the whole mass. I conceive
that such horsebacks running east and west may be compared to
terminal moraines, which, as is is well known, owe their origin to
oscillations of the front end of a glacier pushing forward a mass of
loose materials, thus throwing it up into a transverse ridge, and then
melting away to some point further back." (Agassiz in the Atlantic
Monthly, Feb. & March, 1867). He makes no mention of any
marine fossils occurring in these horseback ridges, and does not seem
to think that the sea was concerned in their formation. I may also
mention that some of these American ridges occur in the region of

Under the name of Osar it, would seem that deposits of more than one kind
have been described, perhaps of different ages and various modes of