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PROFESSOR PRESTWICH ON THE ORIGIN 
both cases projecting shelves having slopes n greater than those of the hill-sides a b* 
Several illustrative instances of these glacial lake-beaches will be found mentioned in 
the works before referred to. 
Fig. 8. 
a, b. Slope of Fill. r. SPore-line or beach, rn. Indent produced by erosion, n (B). Debris removed 
from m. n ( A ). Debris fallen from the slopes or heights above. 1. Lake level. 
§ 11. Structure of the “Roads:” their Inclination to the Horizon. 
The “ roads” are, it is well known, composed of perfectly angular fragments derived 
from the local rocks (mica-schist, micaceous and clay slates,—and granite at the head of 
Glen Roy), with a few rounded pebbles derived in part from the sand and gravel spread 
over the hills by the previous ice-action, or else formed on the spot by the rivulets 
and torrents running down the hill sides. The wear, therefore, of these pebbles is due 
to other and anterior causes, and the angular fragments alone indicate the amount of 
wear attendant upon the formation of the “roads.” It is clear that there was none of 
any moment—none showing long shore wear. 
According to the few positive data we possess, the slopes of the hill sides above and 
below the “roads” vary at different places from angles of about 23° to others of 
about 35°.t Macculloch, who gives the angles of the “ roads,” is silent in respect 
to those of the slopes, the angles given in fig. 9 being only taken by measurement 
from his outline diagrams of the “roads.” In Mr. Brown’s section of the lower 
Glen Roy “ road,” he found the angle of the slopes (measured with a clinometer) to 
be 25°. The Ordnance Survey have, however, determined the exact slopes at a few 
places in Glen Roy, for the following table of particulars of which I am indebted to 
Colonel Parsons, R.E., of the Ordnance Department, Southampton. The references 
are to the 6-inch Ordnance maps. 
* See Sir Charles Lyell’s figure of the beach of the Merjelen Lake, which resembles fig. 8, B, in 
general characters.—‘ Principles of Geology,’ 10th edit., p. 377. There are also beaches in some valleys 
which have been old sea lochs. 
t In one case it is given as only 19°, 
