Although isostatic and eustatic changes(regional and global sea-level changes) were probably the major cause of the eventual breaching of the northwest end of the harbour, there is evidence that regional tectonism also played a role.

Montague Harbour

There were two tectonic events recorded by Mathewes and Clague(1994) that apparently caused subsidence on Vancouver island. At least one of these events is also recorded in the section at Montague Harbour.

The first tectonic event occurred approximately 3600 years BP and is marked by the stratigraphic transition from nearshore intertidal deposits to a subtidal environment characterized by abundant organic matter and foraminiferal fauna.

The evidence for the second tectonic event can also be seen in the stratigraphic record and is marked by a transition from subaerial exposure to a nearshore possibly subtidal environment which suggests a second subsidence event.

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