Tollgate Rock Oil Shale EarthCache
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The purpose of this Earthcache is to give a hands-on introduction to the oil shale layers of the Green River Formation. Access to parking is off of the I-80 service road on the south side of Tollgate Rock. Please make sure you bring a tape measure or ruler to help you with this Earthcache.
Tollgate Rock, like many of the formations in this area, consists of rock from the Green River Formation. The Green River Formation was deposited in a large shallow lake formed during the Eocene Epoch, about 45-55 million years ago. This lake filled with alluvial deposits from inflowing streams, as well as lake sediments, including chemical precipitates like trona, and layers of oil shale. The Green River Formation of Tollgate Rock consists of well-defined layers of greenish brown sandstones and brown oil shales.
Oil shale (not really a shale, but a dolomitic marlstone) is a fine-grained brown sedimentary rock. For millions of years, plant debris and aquatic organisms were deposited at the bottom of the lake. Through the effect of time, pressure, and heat, this material was converted into an organic material called kerogen.
The kerogen in oil shale vaporizes when it is heated to 450-500 degrees C. It can later be condensed to form a synthetic oil which can be refined. The Green River Formation found in the three-state area of southwest Wyoming, northwest Colorado, and northeast Utah, contains vast amounts of oil shale (an estimated 280 billion barrels of oil in Utah alone). However, costs of recovery and waste disposal, as well as air quality concerns, and low-cost imported oil, currently makes it prohibitive in the United States as a fuel source. Just looking at the cliff face from the parking area will give you an idea of the amount of overburden which must be removed to get to the layers of oil shale at this location.
Oil Shale Layers (N 41 32.547 W 109 28.910)
From the parking area, head north to the given coordinates. This is a small sandstone outcrop about 60’ south of the larger Green River Formation cliff face. On this cliff face, you can see two narrow dark-brown bands: one is only a few feet above ground level, and the other is about halfway up the cliff face. These are layers of oil shale.
Walk upwards to the cliff face and look closely at the lower layer of oil shale. Pick up a piece of oil shale that has weathered out of the formation and crush it in your hand. Can you smell the kerogen? Continue walking about 60’ to the northeast along the cliff face to the upper layer of oil shale.
To receive credit for this Earthcache, you must:
1) Measure both layers of oil shale along this cliff face. Send me the width of each layer, and which one (upper or lower) is the thickest.
2) After you have emailed me the requested information, post at least 1 picture of you with your GPS receiver with Tollgate Rock in the background.
The above information was compiled from the following sources:
Lageson, D.R., and Spearing, D.R. 2004. Roadside Geology of Wyoming, 2nd Edition.
Grande, L. 2004. Paleontology of the Green River Formation, with a Review of the Fish Fauna, 2nd Edition.
Oil Shale. Wikipedia. (visit link)
Oil Shale and Tar Sands Programmatic Environmental Impact Statement. Bureau of Land Management. Online at: (visit link)
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