|Upper Branham Lake|
|probably hypersthene (Mg Fe silicate)|
More recent analyses of age dates in southwestern Montana gave rise to another interpretation, by Tekla Harms and her colleagues a few years ago, that the zone through the Tobacco Roots, Highland Mountains south of Butte, and Ruby Range east of Dillon, Montana, represents the old margin of the craton, where a pile of sedimentary rocks formed – possibly during Archean time, but if it was then, it wasn’t long before the 2.5-billion-year cutoff date for the Archean. The sediments might have been early Proterozoic, called Paleoproterozoic. In any case, Harms and colleagues interpret age dates in some of these rocks at about 1.75 to 1.9 billion years to represent the collision between the northwestern corner of the Wyoming Province and another terrane, now mostly in the subsurface of central Montana. There isn’t much doubt that such a collision happened, but there remain questions as to whether the Precambrian metamorphic rocks of southwestern Montana were already there, Archean, or if they were sedimentary rocks that got caught up in that collision and metamorphosed a few hundred million years after they were deposited.
|Geologic Map of part of the Tobacco Root Mountains. Reds and oranges are igneous rocks of the Tobacco Root Batholith, about 75 million years old. Grays are Precambrian rocks, about 1700 to 2500 million years old. Both maps from Vuke et al., 2014, Geologic Map of the Bozeman quad, Montana Bureau of Mines and Geology Open-file map 648. Black box in lower left corner is enlarged below.|
I’m not going to solve the question by walking up to the Branham Lakes. This beautiful location is about 9 miles or so up Mill Creek, east from Sheridan, Montana.
Most of the major valleys on the flanks of the mountains of southwest Montana held glaciers during the most recent glacial period that ended about 12,000 years ago or so.
|Kyanite, Aluminum Silicate|
|Tobacco Root Batholith granite with dark xenolith of older rock|
In the next clip, I made a mistake – I say epidote when I meant to say enstatite. They both start with an E, that’s my excuse! Enstatite is magnesium silicate, and hypersthene, also mentioned in the next clip, is enstatite with iron in it. Both are the kinds of minerals you can get by metamorphosing rocks that have a lot of iron and magnesium, probably NOT simple sediments like shale.
The road to the Branham Lakes, about 9 miles from Sheridan, Montana, is pretty good, and you could probably make it almost all the way in a 2-wheel-drive vehicle if you have decent clearance. I chose to leave my Prius about 2½ miles from the lakes just to be safe, as there are a few dicey stretches, and because it was such a fine day I really preferred to walk. If you go, it would be an unusual year that you’d find the road and lakes snow-free before late June at the earliest, but the setting is spectacular in July and August. I have a few photos from my walk on the blog, history of the earth calendar dot blogspot dot com.
I hope you have enjoyed this little ramble from the Precambrian to the Cretaceous to the glacial period of the Pleistocene. Thanks for listening!
|Lower Branham Lake|
—Richard I. Gibson
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