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Thread: Ice sheet leaves no till

  1. #1 Ice sheet leaves no till 
    Time Lord
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    Cordilleran ice sheet, western edge presumably flowing over southern Vancouver Island. On the mainland coast heavy deposits of till are obvious. There's all sorts of rock dropped there. The land is made of till.

    But just across the Straight of Georgia lie the Northern Gulf Islands, made of sandstone and practically bald. Just bare sandstone bedrock, crumpled and cracked as it heaved up from the sea. What sand and gravel there is, is the same sandstone. Very rarely, one finds an oddity sitting like it was gently placed there.

    Makes no sense. Why would an ice sheet drop crap a hundred meters thick over one place, but further towards its melting fringe deposit nothing? I'm suspecting that the dump site was the ice limit. I'm also suspecting that the rare erratics found on Gulf Islands were floated there by iceburg... if true then I should find none at higher altitude.


    A pong by any other name is still a pong. -williampinn
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  3. #2  
    Time Lord Paleoichneum's Avatar
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    The thing to remember is that the gulf islands, the San Juans etc. were the high points in the gulf and as such the till etc is deposited around the bases of the islands on the floor of the gulf, strait, etc... As the high points the ice sheets plowed over the tops of the island like a massive cheese grater and would not have deposited anything. Any material which would have deposited during the retreat would have quickly washed off in the intervening 10,000 after sheet retreated.


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    Why do rivers scour their valleys down to the bedrock in some places and leave meters thick alluvium in another? There's any number of different factors governing what would be deposited and where.

    Do the rocks show any telltale signs off ice erosion, eg striations? Are they shaped like rocks eroded by ice? If they are currently above water and ice age sea levels were lower, how would ice bergs get up there? just a couple of things to think about.
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  5. #4  
    Time Lord
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    Love the cheese grater analogy - I really see it on the mainland. :-D

    Regarding striations: it's puzzling that no there are none in the Nanaimo/Gabriola Island area. There's a lot of exposed bedrock bent and buckled in line with the subduction (lost in trees you can get bearings just by looking at outcrops). There're also deep faults running at other angles, from contraction I guess. This stuff is relatively soft sandstone, so surely granite dragged SW out of the Rockies should have carved lovely striations.

    Regarding sea levels: it's complicated because this compressable stuff is rising quickly both from plate collision and apparently rebound after ice sheet weight. :? Most of these islands contain lakes and swamps that once harboured ocean species.

    The surface geology of this area, in short, appears impossibly pure and unmarked by glacier.
    A pong by any other name is still a pong. -williampinn
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  6. #5  
    Time Lord Paleoichneum's Avatar
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    Quote Originally Posted by Pong
    Love the cheese grater analogy - I really see it on the mainland. :-D

    Regarding striations: it's puzzling that no there are none in the Nanaimo/Gabriola Island area. There's a lot of exposed bedrock bent and buckled in line with the subduction (lost in trees you can get bearings just by looking at outcrops). There're also deep faults running at other angles, from contraction I guess. This stuff is relatively soft sandstone, so surely granite dragged SW out of the Rockies should have carved lovely striations.

    Regarding sea levels: it's complicated because this compressable stuff is rising quickly both from plate collision and apparently rebound after ice sheet weight. :? Most of these islands contain lakes and swamps that once harboured ocean species.

    The surface geology of this area, in short, appears impossibly pure and unmarked by glacier.
    Two things of of which you yourself note:
    1 Soft sandstone-this means that it is very probable that any striations left have already worn away do to natural weathering

    2. As I noted before you won't get much if any till deposition on the top of a hill I all washes very quickly off to the low spots, in this case the surrounding ocean floor.
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  7. #6  
    Time Lord
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    Thanks.

    Erosion - especially frost lifting - would have been most intense just after the ice sheet: colder climate, no trees to soften wind and rain, no moss to bind the surface.

    I'll look for till in pockets.... wedged in cracks maybe.

    And you're right: the surrounding seafloor is often pebble and cobble, not local stone.
    A pong by any other name is still a pong. -williampinn
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