46 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
geologists, may be summarised as follows : —the lateral streams con¬ 
veyed more material^ such as clays, sands, and coarse gravels, upon 
the valley floors than the greater rivers could convey away to yet 
lozver regions. As a natural consequence of this, the lower area 
of the river basin continued slowly but steadily to rise, owing to the 
accumulation of this material. While this was going on, however, 
the valley plain never ceased to be verdure-clad, for the work of 
the rain and the river was carried on as orderly and as imperceptibly 
as at the present day. There is abundant evidence in the upper 
portion of the Tay valley to show' that as this process w^ent on the 
former position of the river became buried under four, five, or 
perhaps even six hundred feet of such river-accumulated material. 
While this was going on, the valley need never have lost its ver¬ 
dure, nor its beautiful and habitable condition. Indeed, if, as 
I believe there are strong reasons for believing, man were living in 
the valley at that time, he would no more have been aw'are of the 
great changes which the river was then effecting than w'e are to-day 
that these same agents are producing equally important geographical 
and geological changes, though with different results. 
This great building-up of material on the valley floors I have 
elsewhere described as the “Accumulative Period” of post-glacial 
river work.* This accumulative period came to an end, as a matter 
of course, by the superabundant supply of glacial debris becoming 
so far exhausted that the lateral streams then only brought down 
such quantity of material as the river could convey to other regions. 
Then, for a time, a stationary period probably supervened, during 
which the valley floor maintained a comparatively unchanging level. 
In course of time, equally natural conditions followed, which 
entirely revoked the earlier order of the work of the river, namely, 
when less material ivas supplied to it than it ivas able to remove. 
Thus, there came about the denudatian, or un-building, of the great 
masses of alluvium which the river had formerly built up; and this 
change was brought about by the very same functions of the river, 
acting under altered physical conditions. It w^as at this time that the 
floor of the Tay valley began to descend again towards its former and 
its present level, but, in doing so, it left along its hill slopes scattered 
memorials of its former condition, in the form of fragments of its 
ancient high terraces. These fragments we find perched on the 
valley sides at heights of three or four hundred feet above the present 
level of the river. In some places, indeed, as at Ballinluig, they may 
be traced to a height of at least five hundred feet. 
This great dispersion of the contents of our northern valleys w'as 
largely aided by the great upheaval of land surface which is knowm to 
* Geological Society of Edinburgh, 1878. 
