The Eyebrow is one of the archaeological sites covered by Patricia Flint in her excellent UM master's thesis on local archaeology. This quarry is no doubt the reason the creek just to the east is called Flint Creek, and that the Hellgate River was called the Arrowstone by early geographers.
The term "flint" is a slight misnomer, as "flint" is a variety of chert, a sedimentary form of silica, while the silica at the Eyebrow is hydrothermal in origin, and would better be described as chalcedony. Nonetheless, since "Chalcedony Creek" doesn't have the same ring as "Flint Creek", we will refer to the stone at the site as "flint" for historical reasons.
Just to the south of the Eyebrow is Henderson Creek. Apparently a party traveling near the landmark of the Eyebrow ("Arrow Point") tried a pan in the nearby creek, got some gold, and reported it back to Angus McDonald. An account by McDonald (see P. 201) confirms that he assayed gold from Flint Creek prior to 1850, when he reported it to John Owen and some friends - apparently including William Graham - and that the first discovery of gold in what would become Montana was made at Flint Creek. Graham provides the additional information that McDonald then sent out a prospecting party, but they got little done because hostile Blackfeet were in the area. Still, by Graham's account, confirmed by McDonald, it is apparent that some gold was recovered very early in Henderson Gulch, in fact before the gold discoveries in the Bitterroot and Gold Creek.
Below are two maps - a Google Earth aerial view, and a snippet of Lonn et al's 2010 geologic map of the Missoula East quad. In the air photo from Google Earth, you can see why the "Eyebrow" got its name - a patch of timber on the hill indeed looks much like an eyebrow. You can also get an idea of how close the flint quarry at the Eyebrow is to the major placer gold mines along Henderson Creek. The portion of Henderson shown has been extensively dredged. On the geologic map, the Eyebrow is in about the center of the map, in Cambrian sedimentary rocks (Hasmark Formation) with Tertiary andesite and basalt a short distance to the west... the likely source of the hydrothermal fluids that formed the flint.
I infer from the geologic map of the Missoula East quad that the flint at the Eyebrow is chert in a Cambrian formation, such as the Hasmark? Is that correct?
ReplyDeleteI've only picked up flint on the road below the outcrop and haven't been onto the Eyebrow itself. John Maxwell's detailed geologic map published in the MGS guidebook to the area in 1965 has the Eyebrow as Hasmark as well. If it is Hasmark I doubt that the flint is sedimentary (i.e., chert), as the Hasmark is dolomite and shale and really is not cherty where I've seen it. I'm thinking the volcanic rx. to the west may have something to do with generating the flint.
DeleteIn other places I've seen silty carbonates in that part of the section highly silicified by sills intruding them. This article is interesting (I imagine you have seen it) but doesn't really answer the question: http://books.google.com/books?id=jcOqDcDrNt4C&pg=PA65&lpg=PA65&dq=flint+quarry+at+the+%22The+Eyebrow%22&source=bl&ots=kP4dkUfDno&sig=BSZIMMUoPxQPX0RAHnjOGVxQv5M&hl=en&sa=X&ei=maFtUe2AD8OLiwLK6oGACw&ved=0CDAQ6AEwAA#v=onepage&q=flint%20quarry%20at%20the%20%22The%20Eyebrow%22&f=false
DeleteYes, that's the article I link at the very end of the first paragraph. One point of disagreement is that they characterize the outcrop as a klippe and that's not what the mapping shows. I do think your point that silicification is seen to accompany volcanic rocks/sills broadly fits with the most likely origin for the flint at the Eyebrow, given the nearby volcanic rx.
DeleteActually the link takes you to an Alaska page... :)
DeleteFixed! We'll see about adding a piece of the geologic map...
DeleteThanks to this piece and GoogleEarth, I now pay attention to the eyebrow whenever I drive through the area. One day I'll find a way to get closer so I can walk up into those trees. Will I know what I'm looking at? Not likely, but I'll be able to imagine the history that took place there.
DeleteYou take the "Poison Patch" road that turns out of Henderson Gulch and it gets you pretty close. It's on private property and we've opened the conversation on a field trip there this summer in conjunction with a seminar on the mountain man era.
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