There's a bit of info available in a book I've had since my days at CWU, Roadside Geology of Washington. It has a map that points out that in that area, "rock surfaces sloping southwest are mylonite". Mylonites are also found at Yellow Aster Butte, near Marblemount and in the Similkameen.
Have you looked at Geology of the Okanogan Highlands? Some good info there - they refer to the "flat irons" that line the east side of the Okanogan valley between Riverside and Tonasket - is that the same feature?
To me it looks more like a chunk of bedrock that has risen and expanded. The expansion causes faults breaking up the bedrock into blocks. Some of the blocks sink creating the N to S fissures, which are then further widened by weathering. Kind of like a basin and range topography on a much smaller scale.
I agree that the distribution and topography of these features lends itself to an interpretation of basin-and-range expansion faulting. And one of the papers Mike Collins linked to described the formations as "a series of cuestas." I had to look that up, but it is consistent with structural faulting being the cause.
This area did indeed undergo expansion faulting in the past. The problem is that that episode ended 48 million years ago. With unweathered boulders falling off of oversteepened slopes along narrow ravines, I can confidently say that these features are not 48 million years old. They do not seem to be 48,000 years old!
There are numerous exposures of mylonite, as pointed out by Klenk, but again, the mylonite is thought to have formed over 50 million years ago. Mylonite is any rock that has been so sheared by faulting that the original structure is unrecognizable.
Distribution, shape, location, still can't make them all work together in one theory.
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Between every two pines is a doorway to the new world. - John Muir
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