THE GEOLOGY OF PLYMOUTH. 
467 
in the course of untold ages, have been worn into the channels 
through which pass, or once did pass, the waters of the Tamar and 
the Plym. Had not these fractures been made, had the waters 
been set the task of forming their own courses from the beginning, 
the slates would have been an easier prey ; and nature never wastes 
her strength. The gorge of the Plym at Prince Rock has a depth of 
eighty feet below high-water mark, before the rock is reached, 
which there is shale. With slate rocks around, would the lime- 
stone have been thus cut, if the passage had not first been opened ? 
Prom the era of the Trias onward there is a great gap in our 
geological history. There is little evidence of the extent to which 
construction was subsequently exercised over the area under re- 
view. Mighty works were doubtless done, of which we have 
scarcely a trace. Denudation has wiped away countless ages, and 
brought us face to face with these old Devonian times. "We have 
now but the skeleton of the deposits of that far distant epoch. 
Of their Secondary and Tertiary successors we have no remains. 
There is no Lias, no Oolite ; Chalk is very doubtful (flints occur on 
the beaches on the east of the Sound, and on the hill over Staddon, 
but are too slender a foundation whereon to build with certainty 
the superstructure of local Cretaceous deposits) ; no Eocene, no 
Miocene, no Pliocene. 
Quaternary Period. 
This brings us to the confines of the Quaternary system, and to 
the Glacial era. There are in this locality no glacial deposits. I 
am not aware that there are any traces of glacial action, though 
certain alluvial deposits have been, as I hold, mistakenly termed 
drift. There is on the shore of Barnstaple Bay a granite boulder 
which can only have been deposited where it now lies by ice 
action. There are boulders of red sandstone at Waddeton Court, 
on the Dart, which Mr. Pengelly is inclined to believe were trans- 
ported by ice in some form. There are trap boulders at Engle- 
bourne, Harberton, that in his opinion appear to bear the marks 
of ice transportation. The "Head" on Bovey Heathfield has 
glacial characteristics.*' Mr. C. W. Peach observed what he be- 
lieved to be traces of glaciation on rocks in the Dodman district. 
These are the nearest evidences of glacial action to our own locality. 
* " Notes on Boulders," &c, W. Pengelly, f.r.s., " Devon Ass. Trans.," 
vol. vii. pp. 154-161. 
