Thursday, November 12, 2020

Geology of the National Parks in Pictures - Grant-Kohrs Ranch NHS

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


Working our way south from Glacier NP we were able to hit up a couple more of the more isolated national parks in Montana. The first up was a neat little park that preserves one of the larger pioneer open-range cattle ranches in the west. 

Many of my entrance shot pics were taken by my wife and then got deleted in a picture purge without them being sent to me, luckily I do still have some of them.

There are few obvious geological aspects to parks like this one, which focus more on the people than the land. However, the land is a primary reason why these people were out here. Here is a view of the main ranch house. The ranch was first developed by John Francis Grant in 1862, then sold to Conrad Kohrs in 1866 who vastly expanded the ranch. Eventually the ranch reached 10 million acres, however the National Park only preserves 1,618 acres of that property. 

The ranch sits in the Deer Lodge Valley of the Clark Fork River within the greater Northern Rocky Mountains. To the west of the Deer Lodge Valley are the Anaconda and Flint Creek Ranges (seen here in the background). These mountains are composed of Proterozoic, Paleozoic, and Mesozoic sedimentary rocks intruded by Cretaceous and Eocene intrusive and extrusive (volcanic) igneous rocks. Directly at the foot of these mountain ranges is the Anaconda detachment fault. The detachment fault separates the mountains to the west from the Deer Lodge Valley which merges into the mountains to the east. 

Geological cross section of the Deer Lodge Valley. The red arrow denotes the approximate location of ranch. Image courtesy of Foster et al., 2010

The eastern mountains are composed of the Boulder Batholith, which is also the rock that underlies most of the sediment within the valley itself.  The Boulder Batholith is a small batholith on the scale of batholiths, but a major gold producer. In general, a batholith is a very large rock that formed from a magma body deep within the Earth. The Boulder Batholith, named after the boulders that occur as the rock breaks down on the surface, is a large granite body that formed from an igneous intrusion 76 million years ago. The intrusion of the magma produced a hydrothermal system, heating up the groundwater and melting the metallic minerals within the area rocks, including the nearby granite. These metallic minerals, now mobile within the water, were then reprecipitated within the older sedimentary and metamorphic rocks of the area surrounding the granite, producing rick metallic ore veins in conjunction with quartz veins. Not only are these Boulder Batholith related rocks rich in gold, but nearby Butte has one of the richest copper producing zones in the world. Mine run off is actually one of the major concerns for the ranch with toxic chemical polluting nearby rivers and streams.

The valley was formed from similar circumstances as the Basin and Range province to the south and southwest of here. Off the western coast of the US, there was a plate that was subducting (going beneath) North America. That produced compression on the plate and formed the Rocky Mountains. Following the almost complete subduction of the plate the compressional forces on North America were mostly relieved, allowing for the North American plate to expand outwards like a compressed sponge slowly allowed to expand. This expansion process was most noticeable in the Basin and Range province where there are linear mountain ranges alternating with valleys. 

While most of the extensional activity was fairly recent in the Basin and Range province, extensional along the Anaconda Detachment Fault was much older, taking place from 53 to 39 million years ago. This extensional activity forced the Boulder Batholith downwards, while the adjacent mountain ranges moved upwards (in relation). After the downward movement of the valley, sediment started to pile up on the downward block, forming the fertile valley that is present today.


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