Vol. 58.| AMONG THE JURASSIC ROCKS OF SUTHERLAND, 305 
easily consolidate into ice—not by excessively low temperature 
of the sea-water, but—-by ordinary causes. The present mean 
temperature, we must remember, is abnormal; and the isotherm of 
32° Fahr., were it not for the Gulf-Stream, which probably had no 
equivalent in Jurassic times, would almost pass through it, as it does 
actually through Hudson’s Bay and part of Siberia. So small a 
change in the distribution of the temperature would bear no quanti- 
tative relation to a Glacial Epoch, but might bring back an ice-foot 
to fringe our northern shores. 
The process by which materials are collected for transport by an 
ice-foot is thus described * :— 
‘ Subaerial denudation of the surface of the cliffs causes vast masses of material 
to fall during the thaws of the short summer, on a scale so gigantic that the 
mind fails to realize it, unless it has been actually witnessed. The base of the 
eliff is concealed by a talus made up of a shifting mass of material resembling 
those known as screes in the English Lake District... .On the first signs of thaw 
large masses of rock are detached from the cliff, and falling on the [snow- 
covered 7] screes slide down to the ice-foot beneath ; the impetus being often 
sufficient to carry them on to the floe, where they remain until the general break- 
up of the ice, when vast quantities of material are drifted seaward.’ 
To compare this with Sutherland, we have in the high ground of 
Caithness Flags, which accompanies the breccia-beds for the greater 
part of their length, the necessary cliff; and the talus at its base 
may be that seen in Gartymore Burn, where the material which 
reaches the edge of the foot, which melts without much shifting, is 
shown in the bands of breccia of fig. 2 (p. 293). The expression used 
by Prof. Judd in describing the fragments as ‘ angular masses just 
separated from their parent-rock by frosts’ is almost identical with 
part of the above ; and his epithet ‘ wonderful,’ and his description, 
‘a perfect chaos of blocks of stone of the most various proportions 
and of every conceivable shape... . the position’ of which ‘is as 
various as their form and size,’ is a natural picture of the results of 
denudation that must be seen to be realized. 
Again, the building-up of the ice-foot by the accumulation of 
snow to form a soft bed on which the fragments fall, and ever which 
they ultimately siide, cxplains why none of the fragments are 
striated ; while their subsequent mingling with the beach in 
summer will lead us to expect that some of them will be rounded. 
According to our authors, it is not the ice-foot but the ‘ sea-ice 
driven on to shore by gales, or moving up aud down with the tide,’ 
which is ‘ a very potent factor in glaciating rocks and pebbles.’ If, 
therefore, no striated rocks be found, it indicates a temperature only 
low enough for an ice-foot but not for much sea-ice. ‘To proceed : 
‘ The typical aspect of the ice-foot in Smith Sound is that of a terrace, of 50 to 
100 yards in width, stretching from the base of the screes to the water’s edge.’ 
In this we need only change the words ‘ ice-foot in Smith Sound ’ to 
<breccia-beds north of Helmsdale,’ and the description is perfect. 
* Quart. Journ. Geol. Soc. vol. xxxiv (1878) pp. 563 & 565. 
* * Voyage to the Polar Sea’ 5rd ed. (1878) App. p. 340. 
