NATURAL HISTORY OF THE BANKS OF THE TAY. 
47 
have occurred in North-western Europe at this period. In the Tay 
valley we have evidence of this elevation in the fact that in the 
lower part of the strath the deposits which once filled up the valley 
to a great height above the present level of the sea were afterwards 
denuded to a depth of at least a hundred feet belotv sea level. If this 
were so, the only inference is that the land must then have stood at 
least a hundred feet higher than it does now, for the level of the sea 
itself is, comparatively speaking, a fixed quantity. The probability is, 
however, that the amount of the elevation considerably exceeded the 
figure named. 
That the sea did not, and could not, have emptied the lower 
strath of its contents without the aid of such elevation of the land, is 
shown by the fact that after the upheaval and the denudation of the 
valley had been effected, a corresponding subsidence took place, 
which brought the sea once again into this hollowed-out lower valley, 
and when this took place, the sea, instead of further excavating the 
lower valley, was the means of filling it up again with estuarine or 
fluvio-marine deposits. These estuarine deposits, which attain a con¬ 
siderable thickness in the lower Tay valley, and which occupy what 
must have been a high and dry valley floor during the river-denuding 
period of the great upheaval, afford strong evidence of the former 
greater extension of the estuary. 
This subsidence, however, does not end the post-glacial alluvial 
chapter in the geological history of the Tay valley. As we know 
that all around Perth, in lower Strathearn, and in the Carse of 
Gowrie, the plough is driven through the old estuarine clays ; this 
is proof that since the subsidence, which allowed the sea to occupy 
these areas, a minor elevation of the land must have taken place, 
causing the sea to retire once again. Consequent on this second 
upheaval, a further modification of the valley floor took place, which 
brings us at last to the close of this hurried review of the events 
of which the recent or post-glacial deposits of the Tay basin are 
records and witnesses. This last modification was a repetition, in a 
minor degree, of the work which was performed by the river con¬ 
sequent upon the earlier “ Great upheaval,” when such vast quantities 
of glacial and other debris were carried out to what is now the floor 
of the German Ocean. When this last minor upheaval took place, 
the river naturally came down upon the now exposed floor of the 
estuary, and denuded it in proportion to the height to which it was 
raised above sea-level. The evidence of this denudation we may see 
for ourselves in the terraced condition of the land immediately sur¬ 
rounding the city of Perth. 
The foregoing is but a glance at the geological history of the Tay 
basin since the glaciers receded from the Grampians, in the order of 
