The last large glacial advance in the Rocky Mountains started about 25,000 years ago and referred to as the Fraser Glaciation. The main ice centre was in central BC where the ice sheet was over 1 kilometer thick and the glacier flowed in all directions. A glacier flowed southward through this valley during the last ice age, advancing as far as Polson (about 90 miles from Eureka). It retreated northward during the warming and by 11,000 years ago the valleys of southern BC were ice free.
As the glacier retreated blocks of ice calved from the front and these were then covered with debris containing till of boulders, rocks pebbles, clay and sand. This outwash became finer particles the further from the melting outwash. As the ice blocks under this material melted it left a depression i.e. a kettle that then filled with ground water.
Kettles normally have no inlet or outlet but are natural wells fed by groundwater filtered by the silica rich grit free sand of the springs. Lime is leached from the limestone gravel and precipitates as marl on the lake floor. Glacial lakes in the Rockies are known for their turquoise or green colour as they are fed by streams carrying suspensions of fine rock flour. However these kettle lakes are fed by springs not streams, so why the green colour? Most lakes and rivers look blue as water absorbs the longer red, orange, and yellow wavelengths while reflecting the shorter blue wavelengths. However if the water is over light coloured marl versus dark mud the blue is absorbed while the blue-green is reflected, thus these lakes are more green coloured.
Photograph from Ground Zero of the lake bottom of calcium carbonate (lime) marl.
Peckham's Lake is a great place for not only a picnic but also swimming and fishing for Rainbow Trout. It was originally called Norbury, after the magistrate and rancher F. Paget Norbury; but renamed for the Peckham family who purchased surrounding land. While exploring look out for the groundwater springs.
Acknowledgements to: B.C. Pearce (Geological Field Guide to The Southern Rocky Mountain Trench; E Swanson, College of the Rockies; along with diagrams from ©Department of Natural Resources Canada. All rights reserved.
To log this Earthcache, please email the owner with the following: a) the peculiar material you are standing on at Ground Zero b) would you expect the lake level changes to be significant from late spring to late fall? c) Although the lake is on a bowl of sand it does not drain. Why? d) Peckham's Lake is separated from Norbury Lake maybe because?
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