102 MR DAVID MILNE HOME: MEMOIR ON THE 
I also (page 621) hinted at the possible action of the sea upon these block- 
ages, when the sea stood at higher levels. 
This last conjecture has now been strengthened, if not verified, by two 
things—/irst, evidence of the finding of sea shells on Unachan Moor at two 
places, at heights of from 200 to 400 feet above the sea; second, the recogni- 
tion of flats or terraces, apparently marine, at heights from 350 to 450 feet 
above the sea. | 
In order not to interupt my argument, I put what I have to state on both 
of these points in an Appendix (see Notes A. B.) 
Assuming, then, that when the Glen Gloy and Glen Spean lakes existed, 
at heights of 1150 and 856 feet above the sea respectively, the sea was at a 
height of say 500 feet above its present level, what would be the effect of this 
sea action on the detrital barrier ? 
The action would consist not merely of waves and tides, which on a cliff of 
soft materials would be considerable, but also of a current running through the 
Great Glen, now occupied by the Caledonian Canal, a current caused by the 
times of high water being different at the two ends of the kyle or strait.* It 
is also not improbable, that the sea might at this period have had masses of ice 
floating in it. The transport of the immense boulders which now lie high up 
on the mountains here and elsewhere in the Highlands, certainly indicates that 
when the sea stood at heights of 1200 and 2000 feet, it must have had in it 
huge masses of floating ice; and it is no unlikely supposition, that when the 
sea fell to 500 feet, it still had ice init. I need not say how much greater, 
in that case, the effect of a sea current would be in undermining a cliff of 
soft materials composing the supposed lake blockages of Glens Gloy and 
Spean. (See Plate XIV. for the position of these blockages.) 
It is also deserving of notice, that when the sea stood at the greater heights 
above mentioned, there probably was a strong ocean current from the W.N.W., 
because the direction of the parent rocks of the Lochaber boulders leads to 
that conclusion ; and if this oceanic current continued when the sea had sunk 
to the level of 500 feet, the blockages of Glens Spean and Gloy would be 
exposed to the full force of that current. 
VI. Effect of the Removal of the Glen Spean Blockage. 
It this blockage was eroded and undermined by the united action of land — 
streams and of sea, so as to allow of the escape of the waters of the lake, 
what else would happen ? 
The sea would then have free scope to flow up Glen Spean a certain dis- 
tance, till stopped by the rising slope of the land. _ 
* According to Admiralty tide tables, when it is high water at Inverness, it is low water at ions 
William, with a difference of 12 feet between high and low water. 

