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 tare, 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 bv 



*/ o J 



a period of confluent glaciers, when the ice streamed over passes 

 connecting adjoining valleys, leaving in its track lines of moraines. 



