OF THE PARALLEL ROADS OF LOCHABER. 
723 
An analogous effect must have resulted from the gradual melting of the great ice- 
sheet, only that instead of being dependent solely on the seasons and recurrent, it was 
an ever-increasing quantity in consequence of the continuous rise in the annual tem¬ 
perature ; and the waters accumulated in larger and more frequent bodies in 
consequence of the vast extent of the ice-field and the greater compactness of the ice. 
While pools and lakes were formed in the depressions on the ice, rivulets and 
streams on the hill sides scattered sand and gravel on its surface. As the barrier 
ridges melted or burst, the waters escaped to lower levels, carrying with them, on the 
ice,'"' a large portion of this detritus; while at times the circumference of the lakes 
increased until they extended to the hill-sides. Formed at all levels up to 2,000 feet 
or more, these glacier-lake waters in descending to lower levels met and combined 
with other bodies of water, and the transporting forces increased in power till the last 
stage was reached and open channels found in the distant plains—the waters leaving 
as marks of their passage down the valleys, and according to the distance from the point 
of outburst, here great banks of gravel, and there deep beds of sand. Other portions 
of the glacier debris, where the ice became less compact, falling into crevasses or pipes, 
would be carried by sub-glacier torrents, and deposited along the same lower valley 
channels, or swept out to the terminal estuary. 
To these bodies of water pent-up at all elevations on the decaying ice, and to these 
tributary streams carrying down sand and gravel and often casting it in marginal lines, 
may be due many of those numerous terraces and indistinct water-lines observed 
on some of the Scottish hills ; while as other portions of the sand and gravel were 
swept down into the lower levels,—flooded by these great bodies of water, or into the 
sea which we have seen reason to believe then stood higher than now,—the foundations 
were established of those detrital deltas, terraces, and escars, which constitute so 
marked a feature in the valleys and plains of Scotland. 
Besides these forms of drift appertaining to the old ice-sheet, there is the debris 
that would be left in situ on the ground, when the ice melted away. This would 
consist of dirt and rock fragments and boulders, embedded in and scattered on the 
ice, of remnants of moraines, and of small cones of dejection dotted on its sides, or 
formed into lines on its margin, by the streams and rivulets of the surrounding hills. 
Thus we have, on various levels, the transported debris and boulders left by the great 
ice-sheet, together with the angular local debris produced by the intense cold; while, 
commencing in the higher levels, but with a development increasing as we descend to 
lower levels, we have as the result of the action of the rivulets, the streams, the 
glacial-water floods, and the rivers, much of this debris worn and mixed up into an 
undistinguishable mass of gravel and sand, overlying, wherever they remain, the heaps 
of sub-glacial moraine detritus left beneath on the retreat of the great ice-sheet. 
* Streams flowing on the surface of glaciers are found to have a similar power of wear and forming 
pebbles as streams on a land surface. 
