44 TRANSACTIONS-PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
In the foregoing paper I have endeavoured to explain some of 
the rather complicated problems presented by the valley of the Tay 
when considered from the point of view of its geological origin. I do 
not pretend that the explanations I have offered are the only possible 
solutions of these problems. I rather throw them out as suggestions 
for the criticism of other investigators. 
C.— The Superficial Deposits. 
By Rev. F. Smith. 
I have been asked to prepare a brief account of what are techni¬ 
cally described as the “Superficial Deposits” of the Tay Valley. To 
give an intelligible epitome of these deposits in brief space is not an 
easy task; for though they were for many years thought to be so un¬ 
meaning as to be almost beneath the notice of the geologist, they are 
now found to be as full of interest and meaning as any of the groups 
of rocks in the older geological classification. 
The superficial deposits, especially in our northern regions, can 
be well defined from the more solid crust of the earth, as they had 
their origin in a period whose phenomena stand out in goological his¬ 
tory as distinctly as a mountain range from a flat plain,—-namely, the 
“ Glacial Period.” The records of this period have come down to us 
in the form of rounded, scratched, and grooved mountains, and 
almost mountain masses of broken and ground-up rocks, intermixed 
with sands and clavs. These masses of loose material, taken as one 
deposit, form what is known as the “Boulder Clay.” We shall begin 
our review of the superficial deposits, therefore, by endeavouring to 
picture the conditions of the country at the time when the last of the 
glaciers began to recede from their mountain-holds in Scotland. 
The effect of the presence of great ice-masses upon our Scottish 
mountains was the smoothing down of the rugged outlines of the 
latter, and the wide distribution over the country of the material thus 
broken off. All the lateral mountain recesses—that is, all the glens 
and pre-glacial valleys that opened laterally upon the greater water¬ 
ways—became filled with the glacial debris^ and only the greater 
routes of the glaciers were left as open valleys when the glaciers them¬ 
selves had finally receded. Even these greater valleys were irregu¬ 
larly obstructed, and often completely blocked, at their lower ends, to 
a height of several hundreds of feet above the floor of the ancient pre¬ 
glacial river courses. Thus the more ancient configuration of the 
country became greatly modified, and the older rocks were hidden to 
a vast extent under what was known to our geological fathers as the 
“ Diluvial Cover.” This Diluvium^ as it is still sometimes called, is 
known in Scotland as “Till,” and, universally, as “Boulder Clay.” 
