Isostatic reboundIsostatic rebound

Ice moves in, occupies the area for certain period of

time. Then moves out again to leave a depression that has

to rebound/restore to the original height.

Ice moves in, occupies the area for certain period of

time. Then moves out again to leave a depression that has

to rebound/restore to the original height.

Pleistocene glaciation

Cordilleran ice sheet

- 30,000 - 25,000 C yr ago, single body at 20,000 C yr ago.

- divide crest of the Coast Mountains shifted to a position over the British Columbia interior.

- Transition mountain ice sheet to continental ice sheet.

Area Covered

All of southern British Columbia, and into northwestern United States.

Source: Coast, Selkirk and Columbia Mt’s.

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Growth and decay of the Cordilleran ice sheet.

A) Mountain area at the beginning of a glacial cycle.

D) Decay of ice sheet by downwasting; upland areas are deglaciated before adjacent valleys.

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Growth

Data from wood, peat and shell samples.

Within 4000-yr ice increased 2 to 4 fold.

Time period 18,000 to 14,000 C yr BP

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RetreatTerminus of the Puget lobe 100km

back past Seattle within 200-300 yrs.- was helped by calving into proglacial

lakes + sea. - Retreat was coincided with top-down

deglaciation. Became segmented into ice tongues.

- Within 4000 yrs southern BC deglaciated.

By 9500 C yr BP, things are like today (in our area)

Model

The weight applied to the crust is dispersed throughout the lithosphere. The lithosphere is so rigid (like plastic rock) that the weight is transferred across the crust (laterally) resulting in a depression and a bulge in a different place. The lateral displacement of mantle material from below the center of ice sheet loading results in the formation of an area of slight uplift beyond the depression. We can see this by sea-level changes.

The Land

Peak of Glaciation most of British Columbia was depressed due to loading by glacier ice.

Lateglacial shoreline occur on mainland coast up to 200m above present sea level. Depression of 400-500m

The Sea

Places like Parksville show that sea level fell from 108 to 52 m in few hundred years.

By 7000 C yr BP; sea level 5m below present.

Calving embayment in Strait of Georgia

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Uplift

Isostatic uplift migrated in step with retreating glacier margins. It would start as soon as the ice was thin enough.

During Pleistocene and early Holocene.

Some uplift may have been by fault slip.

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References John J. Clague, Thomas S.James. History and

isostatic effects of the last ice sheet in southern British Columbia. Quaternary Science Reviews. 2002

D. P. Dethier, Fred Pessl, R. F. Keuler, M. A. Balzarini, D. R. Pevear. Late Wisconsinan glaciomarine deposition and isostatic rebound, northern Puget Lowland, Washington. GSA Bulletin, 1995.

Tom Rothhamel. Isostacy. 2000. http://www.homepage.montana.edu/~geol445/hyperglac/isostasy1/