Late Quaternary Geology and Geochronology of British Columbia Part 2 : Summary and Discussion of Radiocarbon-Dated Quaternary History
Authors: Clague J J 
Published: February 2014

That period of late Quaternary time in British Columbia for which there is radiocarbon dating control is subdivided into three major units: Olympia nonglacial interval (or Olympia lnterglaciation), Fraser Glaciation, and the postglacial. The Olympia nonglacial
interval probably began more thon 59 000 years ago. It ended with climatic deterioration and glacier growth at the onset of the Fraser Glaciation, perhaps as early as about 29 000 years ago in parts of the Western System and about 25 000 years ago in the Interior System. Glaciers apparently remained
confined to the major mountain ranges throughout the Olympia nonglacial interval, and, in general, the sedimentary deposits, geomorphic framework and processes of the Olympia were similar to those of postglacial time. The Fraser Glaciation began with a build-up of glacier ice in the Coast Mountains.
Glacier growth was slow at first, with ice confined to mountainous regions until 20 000 to 25 000 years ago. The Cordilleran glacier complex attained its maximum size about 15 000 years ago. Subsequent deglaciation was rapid - parts of the coastal lowland of British Columbia were ice free about 13
000 years ago, and the entire province probably was as free of ice as at present before 9000 years ago. During bath the growth and decay phases of the Fraser Glaciation, aggradation occurred in valleys and on Lowlands, drainage patterns were altered extensively, and networks of glacial lakes
developed near glacier termini. Postglacial includes the time from deglaciation until the present. The deposits of this interval formed in response to factors that generally rem a in active today. Extensive aggradation in river volleys during late glacial and early postglacial time was followed by
degradation in most regions. In coastal areas late glacial and postglacial time was marked by changes in the Level of the sea relative to the land. Along the Vancouver Island and main/and coasts, relative sea levels were high during deglaciation due to glacio-isostatic depression of the crust.
Subsequent isostatic uplift resulted in a rapid {ail in sea level relative to the land, such that by 8000 to 11 500 years ago, depending on the locality, inner coastal areas became more emergent thon they are at present. Although these areas remained emergent until recently, deviations in sea level
{rom the present during the last 5500 years have been relatively minor. ln contrast, sea levels on the outer coast (i.e., Queen Charlotte Islands and western Vancouver Island) during middle and late postglacial time were relatively higher thon at present. Differences bet ween the sea level histories
of the inner and outer coasts are due in part to differential diastrophic effects and in part to differences in the timing and magnitude of the isostatic response to deglaciation. The climate immediately following the Fraser Glaciation was cool and moist. In some areas there were minor resurgences
of remnant Pleistocene glaciers during early postglacial time. The climate gradually ameliorated, however, and probably was as warm as or warmer thon the present {rom 8000 to 10 500 years ago until at least 6600 years ago (and perhaps much later). This warm interval was followed in most regions by a
generally cooler moister period which has persisted until present. During this cool interval, advances of alpine glaciers occurred between about 2300 and 3100 years ago, within the last several centuries, and perhaps between 4000 and 5000 years ago. Seven dated postglacial tephras have been
recognized in British Columbia. From oldest to youngest, these are: Mazama, about 6600 years old; St. Helens Yn, 3300 to 3500 years old; Bridge River (older layer), 2300 to 2400 years old; St. Helens P (?), somewhat older thon 2100 years; Bridge River (younger layer), 1900 to 2000 years old; Edziza,
about 1350 years old; and St. Helens Wn, about 450 years old. ln addition, Glacier Peak layer G, approximately 12 750 years old; Edgecumbe (?) tephra, probably 9000 ta 11 000 years old; and White River tephra, approximately 1200 years old, may occur in British Columbia, although they have not been
identified there.