OF THE PARALLEL ROADS OF LOCHABER. 
725 
4th. In the Lochaber district, while the exceptional accumulation of ice in the Spean 
Valley heavily barred the entrance of the glens on the north side of that valley, the 
passes at the head of the glens were also blocked by smaller remnants of the great ice- 
sheet ; and the formation of the detrital shelves or terraces is due to the sudden 
bursting of these minor barriers, when the waters of the lake were discharged with 
great rapidity until they fell to the level of the col. Under these circumstances the 
mass of loose debris covering the hill-sides gave way and slid after the retreating- 
waters, until stayed with greater or lesser abruptness, according to the angle of slope 
and the volume of the mass, on the discharge ceasing and the waters coming to rest. 
The shelves so formed, modified slightly by subsequent subaerial action, constitute 
the “roads.” 
5 th. The moraine or sub-glacial detritus, in places where the glaciers clashed and 
their progress consequently became checked or delayed, tended also to accumulate or 
heap up, and, in this way, added in the Lochaber glens to the strength and permanence 
of the ice-barriers. 
6th. While the moraine detritus was irregularly distributed under the ice, or massed 
in particular places, the debris projected on the surface of the ice-sheet and contained 
in its body was either, as the result of the great floods consequent upon the bursting 
of pool- and lake-barriers, carried successively to lower levels, leaving here and there 
banks of sand and gravel at various heights on the hill-sides, or else was left in situ on 
the liquefaction of the thick mantle of ice. These destructive floods, combined with 
the unceasing river inundations due to the same general thaw of the great ice-sheet, 
carried down and spread out in the valleys and plains the great beds of gravel and sand 
which, with the modifications since brought about by subsequent continued fluviatile 
action," 7 have given rise to various forms of escars, terraces, and other less defined 
accumulations of these detrital materials. 
Explanation of Plate. 
The topographical details and the range of the parallel roads are reduced from the 
shaded one-inch Ordnance Map. The probable positions of the Spean Valley and 
Glen Roy barriers are on the same authority. That of Glen Gluoy is conjectural on 
the part of the author. The heights are taken from the one-inch contour map of the 
Ordnance Survey. 
In the names of places the spelling of the Survey is usually followed ; but a few 
other names, commonly used by previous observers, have been retained. The direction 
* The terraces and loose deposits due to this cause in the south of England and north of France will 
be found described in a former paper by the author—Phil. Trans, for 1864, p. 247. 
