20 On the Origin of the Parallel Roads of Lochaher. [May 1 



ensued which led to an exceptional heaping up and accumulation of 

 the ice in front of Grlen Koy and Grlen Grluoy. 



2nd. In consequence of the lowering and partial submergence of 

 the land and its conversion from a continental area to an archipelago, 

 combined possibly with some other more general cause, an ameliora- 

 tion of the climate took place attended by a gradual melting of the 

 ice-sheet in all the lower tracts. The snow and ice wasted from the 

 valleys and from the lower mountain summits, and in the absence of 

 any established water-courses, the hollows and depressions in the ice. 

 when not fissured, were converted into pools and tarns, until the con- 

 tinued liquefaction opened out surface channels or interior fissures, 

 by which the water could ultimately escape. 



3rd. That pending the establishment of natural lines of drainage, 

 and in presence at places cf unusual obstruction, the water accu- 

 mulated in some valleys in larger bodies or lakes ; and if in those 

 cases, the mouth of the valleys being closed by the main ice- barriers, 

 other channels of escape such as cols or passes communicating with 

 adjacent glens or valleys presented themselves, the water overflowed 

 through these channels as soon as it rose to the height of such cols or 

 passes. Or should the cols have been also barred by ice. that ice 

 would have given way as soon as the increasing height and pressure 

 of the water proved sufiicient. When this happened, the water would 

 at once fall to the fixed level regulated by the col. and thus no record, 

 such as we have in Lochaber, might be left of the presence of the 

 original bodies of water, 



4th. In the Lochaber district, while the exceptional accumulation 

 of ice in the Spean Valley barred the entrance of the glens on the 

 north side of that valley, their passes were also blocked by remnants 

 of the great ice-sheet ; and the formation of the detrital shelves 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 below the line of water level, 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." 



5th. The moraine detritus in places where the glaciers clashed, and 

 where their progress consequently became checked or delayed, tended 

 also to accumulate or heap up, and in this way in the Lochaber glens 

 added 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-stream and contained in its body, was either left in 



