724 
PROFESSOR PRESTWICH ON THE ORIGIN 
But however wide the bearing of the conditions which existed in Lochaber may be 
upon the complex general phenomena of the drift deposits of Scotland, it is too small 
a standpoint from which to discuss that great question. I have therefore here restricted 
myself mainly to a general consideration of those problems that hinge more especially 
upon the remarkable conditions which, while they are wide spread, show themselves 
with exceptional distinctness in the Lochaber district.'"' 
§ 17. Conclusions. 
The general conclusions I draw from the phenomena in Lochaber and surrounding 
districts are— 
1st. That at the period of the first great glaciation of Scotland the ice-sheet in 
Lochaber attained a thickness of not less than 2,000 to 2,500 feet, and that, in conse¬ 
quence of the peculiar physiographical conditions of the district, the large ice-streams 
from Ben Nevis so clashed with others in the Spean and Caledonian Valleys that a 
block ensued, which led to an exceptional heaping up and accumulation of the ice in. 
front of Glen Spean, Glen Boy, and Glen Gluoy. 
2nd. In consequence of the lowering and partial submergence of the land, and its 
conversion from a continental area to an archipelago, combined with some other more 
general cause, an amelioration of the climate took place, attended by a gradual melting 
of the ice-sheet. 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 were converted into pools and tarns, until the continued 
liquefaction opened out surface channels or fissures by which the water could ultimately 
escape. 
3rd. That pending the establishment of natural lines of drainage, and in presence, at 
places, of unusual obstruction, the water accumulated in some valleys in larger bodies 
or lakes ; and if, in such cases, the mouth of the valley being closed by ice-barriers, 
other lower channels of escape, such as cols or passes communicating with adjacent open 
glens or valleys, presented themselves, the water overflowed through those channels 
a,s soon as it rose to the height of those cols or passes. Or should the cols have been 
also barred by ice, that ice would give way as soon as the increasing height of the 
water gave sufficient pressure, or led to an overflow. When this happened, the water 
would at once fall to the fixed level regulated by the col, and a definite line of water- 
level would be established, which would continue as long as the main barrier at the 
entrance of the valley lasted. If, however, this main barrier gave way before the 
pent-up waters found an escape by the pass, then, in the absence of any other outflow, 
as the lake would be always filling and never remain long at the same level, no permanent 
line of water-level could be established, and thus no record, such as we have in Lochaber, 
need have been left of the presence of such bodies of water. 
* See on tliis subject 4 The Last Stages of the Glacial Period in North Britain,’ cit. ante. 
