This week’s feedbacks are both answered by geologist Dr Emil Silvestru, CMI-Canada. First, Andrew R. from New Zealand writes:

I am having trouble understanding how such features as sharp curves in canyons, such as “Horseshoe Canyon”, formed rapidly. Searches for this topic lead to, “slow, gradual, meandering” etcetera as an explanation. Please, I trust there must be an alternative “Biblical age” explanation and am hoping you can provide me with it.

Dr. Silvestru replies:

Dear Andrew,

Thank you for this quite interesting inquiry. The subject of meandering is a much more complicated one than generally acknowledged in the geology literature. The vast majority of textbooks and treatises leave out meandering in hard rocks, especially meandering in caves. River meandering is a fairly straightforward process and it is almost certainly and exclusively caused by flow dynamics and sediment transportation. This meandering is practically 2-dimensional.1 In caves, 3-dimensional meandering is much more ample, the vertical component being sometimes on par with the horizontal one.

With the above in mind, slow meandering over long ages in hard rocks becomes quite a problem. In order to avoid the difficulties, they almost always avoid discussing vertical meandering, which is not easily explainable by slow dissolution—chemical and mechanical erosion also don’t explain it.

If you can, please check this image out: http://www.newswise.com/articles/drainage-of-subglacial-lakes-created-canyons-of-antarctica-12-14-million-years-ago (you can enlarge it quite a lot) you will notice that confined flow (between the bottom of the ice sheet and the bedrock) can create significant meandering in very hard rocks during catastrophic events. If you have a look at this video: http://www.youtube.com/watch?v=8LQIuuDSDJE you can see how wonderful and elegant sharp bends were carved in hard rock by glacial meltwater on Kelley’s Island in Ohio, US. Catastrophic drainage again—the subglacial flow created the grooves. Splendid examples of meandering in hard rocks are also found in the Channeled Scablands in Washington State which were the result of at least one catastrophic glacial meltwater release from Lake Missoula….

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