My next series of posts about the Geology of the National Parks Through Pictures is from a trip we took over the summer of 2017 up to Canada and back down through Montana to hit a bunch of the glacial parks in the area. These include two Canadian National Parks.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
Grant-Kohrs Ranch NHS a little to the northeast. As can be seen on the map below, on the west sits the Anaconda Range and the Anaconda Detachment Fault. These mountains are composed of Proterozoic, Paleozoic, and Mesozoic sedimentary rocks intruded by Cretaceous and Eocene intrusive and extrusive (volcanic) igneous rocks. To the east sits the Pioneer Range, composed of the Pioneer Batholith (PB on the map below). The Pioneer Batholith is very similar to the larger nearby Boulder Batholith, being emplaced ~70 million years ago during the Late Cretaceous. The batholith was formed from magma that intruded into the area during the mountain building that resulted from the compression of the North American plate by the Farallon Plate off the west coast of the US squeezing the western portion of North America (called the Laramide Orogeny). One of the largest plutons (a smaller body of rock formed from magma within the batholith) within the Pioneer Batholith is the Late Cretaceous age Uphill Creek Granodiorite, which covers much of the Pioneer Range.
|Map of the regional geology of Big Hole Valley. Image courtesy of Foster et al., 2010.