Landforms of British Columbia 1976 - Department of Geography

materials, with a deeper basin on the inner side. Soundings along the fiords are
not everywhere available, but depths are mostly greater than SO fathoms. The deep-
est recorded sounding in British Columbia waters is 418 fathoms* (&SOS feet)
in Finlayson Channel on the east side of Swindle Island. This is comparable to the
remarkable sounding of 483 fathoms (2,898 feet) in southeastern Alaska at the
southern end of Chatham Strait.
The fiord system is a product of intense glaciation of a mountainous coastline.
Pre-glacial valleys on the western side of the Coast Mountains were occupied by
Pleistocene ice and served as escape routes for ice flowing westward to the sea from
the high area of accumulation along the crest of the mountains. The moving ice
eroded the valley walls to their present steep profiles, and the great thickness of
ice eroded the bottoms of trunk valleys to considerable depths below sea-level.
Then, as the ice left the valleys they were invaded by the sea to produce the drowned
fiord system of today.
SEA-LEVEL CHANCes
Changes in the relative height of sea-level have taken place along the western
margin of British Columbia continuously during the geologic past, and as a consequence
the position of the coastline has changed constantly.
During Tertiary time no marine Tertiary sedimentary rocks were deposited in
British Columbia except in basins between the Queen Charlotte Islands and the
mainland, at the mouth of the Fraser River, and as coastal plain deposits along the
west coast of Vancouver Island. This implies that the mainland stood well above
sea-level and that the coastline during the Tertiary remained in much the same
general position as it is now. In the same period there were changes in the height
of land above sea-level. The Tertiary opened and closed with uplifts which re
juvenated the erosive power of the rivers. The effects of the uplifts are expressed
topographically by many features along and inland from the coast. Despite the
reduction of the land surface by erosion, the land stood relatively higher at the close
of the Tertiary than it did at the beginning.
Changes in sea-level have continued to take place during and since the Pleistocene.
They are not well known, are variable from place to place, and are complicated
by the fact that they involve the interplay of three factors: (a) isostatic
adjustments of the crust, (b) tectonic movements of the crust, and (c) eustatic
mOveme”ts of sea-level.
Isostatic adjustments of the earth’s crust took place in response to its loading
with the great accumulation of Pleistocene ice and its unloading with the subsequent
melting. The ice load depressed the land, which subsequently rose as the ice finally
melted. It is generally considered that the isostatic readjustment of the land was
nearly contemporaneous with the melting of the ice load, and that today equilibrium
has been reached. The total isostatic depression of the land due to ice load has
been estimated to have been at least 1,000 feet in the Fraser Val1ey.t
Tectonic movements of the crust in response to mountain-building forces may
produce either uplift or depression of the land. Tectonic movements in the Haines
area of Alaska produce a rise of the land of approximately 0.08 foot per year as
determined by studies of tidal gauges. No conclusion can be derived from gauge
records in the vicinity of Vancouver or Victoria.
Eustatic changes of sea-level took place during the Pleistocene as glaciers grew
or diminished and water was withdrawn from the ocean or returned to it with a
corresponding fall or rise of sea-level. The eustatic rise of sea-level since the
a