A Giant, Geological Layer Cake


All sedimentary rock, by definition is laid down in layers, strata to use the geological term. Often the layers get shoved around, folded and twisted, as they move through the tectonic grinder. It’s rare to see a mountain, a very large mountain, composed of apparently unaltered strata.

Meet Mount Robson.

Mount Robson, from Mount Robson Provincial Park Visitor Center, British Columbia

Mount Robson, from Mount Robson Provincial Park Visitor Center, British Columbia

It’s a 12,969 foot tall layer cake. The strata appear to be very nearly level, just uplifted almost 13,000 feet. The buff-colored middle layer is shale; it separates the top layers of limestone and dolomite from the lower levels of quartzite.

But the apparently level layers are an illusion, an artifact of the sheer scale of the geology of this part of the Canadian Rockies.

Its rock layers are part of a huge syncline or downward fold in the rocks. Its location near the bottom of this syncline causes the layers to remain horizontal in appearance. The summit is made of carbonates (limestone and dolomite). A band of buff coloured shale separates the upper carbonates from the lower section composed largely of Gog quartzites.

Imagine a vast, flat plain of sedimentary rocks, laid like a carpet on deeper strata. Now push one edge of the carpet. The carpet develops a series of sinusoidal curves. Looked at in cross-section, you’d see a mess, like Mount Fitzwilliam

Mount Fitzwilliam, Jasper National Park, Alberta

Mount Fitzwilliam, Jasper National Park, Alberta

If you study the strata of Mount Fitzwilliam, which is quite close to Mount Robson, the geology will give you a headache. Portions of the summit are composed of strata stood on end, rotated 90 degrees by tectonic forces. to the east (left), the strata are inclined about 60°. The western (right) strata are nearly vertical but sliced by bands of quartzite at a 45° angle. Geology is messy; this is what happens as the sinusoidal curves get mashed together. Some curve even flop over on top of others, creating the misch-mash you see here.

Mount Robson looks so pretty because it is at the flat bottom of one of those sinusoidal curves, and you are looking at it end-on. The curves – called synclines and anticlines – are so wide the Mount Robson looks undisturbed.

Endless Mountain, Jasper National Park, Alberta

Endless Mountain, Jasper National Park, Alberta

Here you can see a chunk of a sinusoidal curve, modified by glacial erosion of the softer shales. The summit rock is that same limestone and dolomite – ancient sea bottom – curved down toward you a bit; the shales are obliterated to talus, but below them you can see the quartzite starting to curve back up. It’s called Endless Mountain because it goes alongside the road like this for many miles. Which gives you some idea of the scale of events when the Canadian Rockies were made.

WC apologizes to any Canadian geologists wandering to this blog for the gross oversimplifications. Yes, there are multiple faults, lots of overthrusts and the details of the rock layers vary a lot.

For sheer scenic value, Jasper National Park approaches – just approaches, might you, it doesn’t surpass – the scenery of Alaska. And the geology, as you can see, is very cool.

 

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  1. Pingback: A Giant, Geological Layer Cake | Was. Is. Is to come.

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