During the decay of the Cordilleran Ice Sheet 10 000 to 13 000
years ago, glacial lakes developed within valleys that dissect the
Interior Plateau of British Columbia.
Glacial Lake Thompson and Glacial Lake Deadman formed due to ice
dams. Both Glacial Lakes were ribbon-shaped, deep lakes that
contained significant water volumes. Water flowing into the lakes
carried fine silts and clays, and deposited them in distinct annual
layers on the lake bottom. The clays stettled in the winter when
the lakes were frozen and the water was calm. The silts settled in
the summer. The slit and clay layers are called varves and
represent one year's deposition. The thickness of the summer layer
will vary from year to year as the amount of meltwater and sediment
input to the lake are dependent on snow levels, temperature,
rainfall and other natural factors. The photo below show an example
of a the layers of varves that can exist in sedimentary
formation.
In time the ice dam broke, allowing the water to flow down the
valley and draining the lake, and re-establishing the South
Thompson River.
The South Thompson River then cut the current valley through the
soft silt, leaving the old lake floor high and dry, as benchlands
of silt.
The silt and clay deposits left by the Glacial Lakes are cut and
carved by flowing water leaving very distinctive silt terraces and
cliffs. These features can be seen today in many parts of the
Fraser River valley, in the Okanagan valley around Summerland and
Penticton, and in south end of the Kootenay valley.
The manner in which homes are built and where people choose to
live has been directly affected by this geological feature.
To log this earthcache email the answers to the following
questions to the cache owner. Please do not post the answers you
may post a picture but it is not a requirement:
1. Estimate the average thickness of the varves visible on the
bottom half of the silt cliff.
2. Provide a brief explaination on why you think the varves are
more visible in this section and not as visible in the silt cliffs
across the river.
3. Provide an explaination for why the varves are different
widths from the bottom of the cliff to the top.
as per the new guidelines any posting of photographs is
optional
This cache was a collaborative project between the cache owner
and an inquisitive student from Bert Edwards Science &
Technology School.