July 26, 2023  

Spokane, Washington, to Gillette, Wyoming: Geology in the Rearview Mirror

This post is experimental and not particularly interesting, but it is the best I can do under the circumstances; I followed Interstate 90 through the Rocky Mountains at 70 mph, with no pull-offs, and trying to take photos in the heavy traffic would have been suicidal. Instead of including a map, photos of outcrops, and some close-ups to examine mineralogy, I am relying on geological maps and my memory. The most experimental part is that I’m working on an iPad, which is a blessing and a curse. Let’s see how it worked out.

Figure 1. I got this image from Wikipedia because no one takes photos of road cuts. All I could find were scenic photos like this, which are useless for my purpose. Nevertheless, the steepness of the peaks gives some idea of why I didn’t stop. The road cuts showed sedimentary rock layers tilted every direction. This is the first mountain range east of Spokane, but the next dozen or so were similar in form.
Figure 2. This geologic map from RockD shows the rocks I encountered between Spokane and Gillette, Wyoming (shown by the blue dot in the lower-right). The volcanic rocks I saw in the Spokane area (last post) didn’t cover the Bitterroot Mountains (purple area between the two leftmost circled areas). The purple rocks are fine-grained metamorphic sediments between 1600 and 1000 my old. The second circle roughly outlines the area where Lake Missoula formed, dammed by ice during the Pleistocene. The steep and resistant (to glacial erosion) Precambrian rocks formed channels that were easily blocked by ice. When the glaciers retreated periodically, huge floods escaped through these passes. I drove though some of them without knowing it (I was busy). Riverside Park, where I found anomalous boulders, is one such episodic flow path.
Figure 3. This map segment reveals several Cretaceous batholiths (pink hues) east of the Bitterroot Mountains. I circled one in this map that I drove over when leaving Butte, Montana on I-90 this morning. The litho logo was easy to identify from the truck at 65 mph, even with all the tight turns.
Figure 4. This map segment identifies some of the oldest rocks in North America (marker at lower-left of image), which are as old as four-billion years. For reference, Sheridan is circled (see Fig. 2 for its location within the larger region. The area covered by the inset comprises nearly horizontal Miocene (~50 my) sediments that formed cones resembling small volcanoes. This nascent “badland” was at least 60 miles across.
Figure 5. This geologic map shows my route (black line) along Interstate 90, from Portland, Oregon, to Gillette, Wyoming, with stops in Spokane, Washington, and Butte, Montana.


Summary. The oldest rocks (Precambrian metasediments shown in purple shades) are scattered throughout the Rocky Mountains. These old rocks were pushed and pulled for hundreds of millions of years as microcontinents collided to form what we call western North America.

Paleozoic rocks (500-230 my) that would have been deposited on top of them, or intruded into them, are only found in scraps here and there (I’m speculating, but Paleozoic rocks have a habit of turning up in the unlikeliest places).

During the late Cretaceous (about 80 my), granitoid intrusions forced their way into these older rocks, as I saw at Butte and other small mountain ranges (pink and tan hues). This was a geologically active period in the evolution of western North America.

About fifty-million years ago, volcanoes formed along the western margin of North America (e.g. Mt Hood and other volcanoes produced thousands of feet of volcanic rock, forming the Columbia plateau (yellow shades in Fig. 5). At approximately the same time (50 – 5 my) sediment was collecting in lakes and shallow inland seas leftover from the Cretaceous Interior Seaway. These sediments are undeformed and not very well lithified (i.e. not buried deeply); they appear east of Bozeman MT as green in Fig. 5.

Hidden beneath the Precambrian rocks, which were pushed eastward as much as 150 miles in Canada, and Miocene sediments, lay the oil and coal rich Cretaceous sediments laid down between about 150 and 60 my ago. As proof of this, Billings MT (rightmost circled area in Fig. 2), with a population less than 150 thousand, has three oil refineries; but it is so remote, with so little infrastructure (e.g. pipelines), that trucks deliver refined petroleum products to rail cars. It is a modern western boom town.

We’ll see what tomorrow brings …

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