134 
BATHYMETRICAL SURVEY OF 
along the crest of the arch, the higher members of the lower division 
were worn away, and the volcanic rocks were laid bare, before the 
Upper Old Red Sandstone was deposited. The members of the upper 
division occur near Bridge of Earn, and extend beneath the estuary of 
the Tay and the Carse of Gowrie to near Dundee. Between Forgan- 
denny and Bridge of Earn, the basement beds are found resting 
unconformably on the denuded Lower Old Red Sandstone volcanic 
rocks, where fragments of the latter occur in the breccias. On both 
sides of the estuary of the Tay, however, the Upper Old Red Sandstone 
is brought into conjunction with the volcanic series of the lower division 
by two parallel faults. The members of the upper division are 
composed mainly of friable brick-red sandstones and marls, which have 
yielded near Errol fine specimens of the genera of fishes characteristic 
of this division. 
Reference has already been made to the fact that a small patch of 
Carboniferous rocks appears about half a mile to the south of Bridge of 
Earn, which are brought into conjunction with the Lower Old Red 
volcanic rocks to the south by means of a fault. The strata consist of 
blue clays and shales, sandstones, and calcareous bands belonging to 
the Cementstone group. Small though it be, this remnant is of great 
importance in proving the former extension of the Carboniferous rocks 
over the lower part of the Tay basin, from which it has been almost 
wholly removed by denudation. 
The existing valley system of the basin of the Tay furnishes 
admirable examples of the influence of geological structure in 
determining the direction of the water drainage. The upper part of 
the Tay itself, and many of the tributaries within the metamorphic 
area, flow approximately in the direction of the strike of the crystalline 
schists. The massive Ben Ledi grits, the Green Beds, the sills of 
epidiorite and hornblende-schist, and the Perthshire quartzite have 
each had a powerful influence in the development of the prominent rock 
features of the region. Where these occur in association with zones of 
mica-schist and phyllite, they have more successfully resisted erosive 
action, and have given rise to rocky barriers or precipitous escarpments, 
thereby contributing to the formation of gorges, and in some cases of 
rock-basins. 
The evidence relating to the glaciation of the Tay basin leads to the 
conclusion that, during the climax of the Ice age, the region must have 
been covered with one continuous sheet of ice, the movement of which 
must to some extent have been independent of the existing valley 
system. Where the rocks have been able to retain the striae, the latter 
have been found up to elevations of 3000 feet, showing that the highest 
mountains were over-ridden by the ice. This stage was followed by 
a period of confluent glaciers, when the ice streamed over passes 
connecting adjoining valleys, leaving in its track lines of moraines. 
