The northern part of this parish rests on granite, and it approaches
very nearly to the church. Proceeding southward, the granite is
succeeded by the same kind of slate as that which is found in the
adjacent parish of St. Austell, both parishes lying parallel to each
other, in respect to a line extending from the granite hills to the
sea shore, have the same geological structure.
It is known that within the memory of the last generation the sea
flowed up to St. Blazey Bridge; and various indications of its having
reached half a mile further up the river have been detected: thus
showing that the sea, which has encroached on most parts of the coasts
of Cornwall, has at the same time been driven back from the land. This
effect is usually attributed to the accumulation of detritus brought
down from the hills by rivers, more especially when they are in flood,
and undoubtedly this must be a generally operative cause; but in this
particular instance the effect must be mainly ascribed to the wearing
away of the diluvial sand-bank at the head of the adjoining bay. The
volume of this river, and its consequent momentum, not having been
sufficient to counteract the deposition of sand by the waves running
up the estuary, with which, in their rapid motion, they are always
charged.