the Parallel Roads of Glen Roy. 31 
or ice-cap, so deep that its weight squeezed icebergs out of the 
valleys into the sea as fast as the snowfall accumulated upon its 
surface. Then slowly the Ice-age began to abate; it got warmer, 
and more snow was melted in summer-time than formerly. That . 
does not mean that the ice-cap began to get thinner all over, 
for the rainfall and snowfall is much greater in the west than 
towards the east—e.g. at one end of the Caledonian Canal the 
rainfall is well over 100 in., at the other end it is only 25 in. 
per annum; hence, clearly, while the yearly thaw was between 
those amounts, in the one place the ice and snow would be 
melting right away, while in the other, though just as much 
thawed each year, the ice-cap would be kept at its full thickness 
by the excess of snowfall upon it. 
The prevailing south-west wind is the chief rain-bearing wind ; 
this wind, after skirting the North of Ireland, is caught by the 
Firth of Lorne and conducted by Loch Linnhe to Ben Nevis; 
hence the snowfall there would be very great. Even nowadays 
the annual rainfall is as much as.80 in. at Fort William, 100 in. 
on the hills, and 120 in. in Glen Coe, and as much as 146 in. on 
Ben Nevis, while in Glen Roy it is only 40 in., and 46 in. at 
L. Laggan. In the Ice-age the precipitation on Ben Nevis was 
even greater, but the chief difference then was that much of the 
rain which now falls on the south-west slopes of the mountain 
was then, as snow, blown over Ben Nevis and drifted into the 
Spean Valley under its lee, while at Glen Roy and Laggan the 
rainfall was less than at present; hence for a long time after the 
ice and snow was melted away from the upper end of the Spean 
Valley, from Glen Roy, Glen Gloy, and beyond, the last remnant 
of the glacial ice-cap, sheltered behind Ben-Nevis, remained un- 
impaired in the mouth of the Spean Valley. 
I have marked on the contour map of the district the position 
which, I suppose, this huge snow-drift once occupied. (Fig. 6.) 
It will be seen this ice-mass blocks up the mouths of Glens 
Gloy, Roy, Glas Dhoire, and Spean, so as to dam back lakes in 
them, each overflowing at its ‘‘col”’ at the far end of the glen. 
This state of things had existed for a long time before the ice- 
mass had shrunken to this size, and so the highest roads were 
formed. Now, although the warmth of the sun and air had 
been insufficient to melt this obstruction, it is clear that the 
waters of the lakes would have more effect, for they gather 
warmth all along their valleys, and expend it in melting the 
ice-block where they touch it. So each of the lakes is drawn 
with its corresponding gulf hollowed out of the ice. This gulf, 
in the case of Glen Gloy, for instance, will go on extending and 
undermining till it reaches the second position drawn on the 
map. A little more of this action, when it has reached the second 
ice-margin drawn in the-map and shaded diagonally, and Lake 
