716 
PROFESSOR PRESTWICH ON THE ORIGIN 
the period of submergence which succeeded the great glaciation, the question arises 
what was the height at which the sea stood in this area at the time when the Lochaber 
lakes existed ? 
Section of the Spean Valley adjacent to the line of barrier. 
Fig. 23. 
Section 1 mile further west. 
The levels of 1,000 feet or of 1,250 feet show the height needed for the barrier. 
The lake is represented by the broken lines in fig. 22. 
As I see no sufficient grounds for attributing the origin of the erratic blocks and 
detrital terraces at high levels to marine action, and as it is possible to account for 
many of even the lower terraces in Strath Spean by causes in connexion with the 
Spean glacier lake, there seems to me no reason why the submergence in Lochaber 
should have exceeded 300 to 500 feet. In the valley of the Spean the terraces are of 
very variable height, ranging, according to Chambers, in the lower part of the valley, 
from 325 feet at Tiendrish, 283 feet at Inverroy, and 345 feet at Inch, to, higher up 
the valley, 392 feet at Auchaderry, 446 feet at Monessie, and 627 feet on the side of 
Craig Dhu. Nor do the levels of the terraces on opposite sides correspond. These 
irregularities of horizon are not in accordance with lines of permanent water level either 
of lake or sea. Nevertheless some of the terraces described adjacent to this district by 
Chambers below the level of 300 to 400 feet, may possibly be due to marine action.* 
In the absence of more positive data, it is, however, with some hesitation that 
I express an opinion that whatever may have been the height of the sea in other areas, 
it stood during the period of maximum submergence in this particular district at not 
more than about 500 feet, if so much, above its present level. 
* He states there is a terrace 325 feet high at Fort William, which corresponds with the height of the 
one at Tiendrish, while around the first place he mentions nine others below that level (the lowest being 
32 feet), of which six, between 288 and 96 feet, correspond very closely with others at Inverness and 
Fort Augustus on the Morayshire coast. The coincidence of these is sufficiently marked to render it 
possible that they may all be referable to a common cause such as a sea-level, as the land rose again after 
the submergence. Allowing, again, for different degrees of elevation at distant places, might not some 
of these last terraces correspond with the higher beaches of Jura and adjacent islands? 
