Geologic Trip, Southeast Oregon

Steens Mountain

USGS

Head of Little Blitzen Gorge

Text Box: Catlow Valley

Steens Mountain (9773’) is a large mountain in southeast Oregon that has characteristics of two major geologic provinces that overlap in this area—the Basin and Range Province to the south and the Columbia River Basalt Province to the north. The mountain is formed from a large fault block of basalt that has been uplifted and tilted to the west. The fault block is part of the Basin and Range Province and the basalt is part of the Columbia River Basalt Province.

 

The east side of the fault block is a steep rugged fault escarpment that rises over 5,000’ above the Alvord Desert. The west side is a gentle slope that extends for about 20 miles to the Catlow Valley and Frenchglen. At high elevations the gentle western slope is dissected by several gorges that were dug by glaciers during the Pleistocene ice ages.

 

The Steens Mountain Backcountry Biway goes up the gentle western slope of the mountain and provides access to the crest. This 52-mile gravel road consist of two segments. The north loop extends from Frenchglen to the crest of the mountain and is a good gravel road suitable for passenger cars. Along the way there are good views of the Kiger and Little Blitzen Gorges. The south loop has segments that are rough and steep and is not suitable for low-clearance cars or trailers. The loop road is closed in the winter due to snow, and may be closed by weather at other times.

The photo looks north along the crest of Steens Mountain and shows the north loop road near the head of Little Blitzen Gorge. The gorge is on the left and the east escarpment of Steens Mountain on the right. In this area, the loop road will eventually be cut by eastward erosion of the Little Blitzen Gorge and by westward erosion of the eastern escarpment. Note the gentle west slope of the top of the fault block. 

The photograph looks west down Little Blitzen Gorge from the head of the gorge near the loop road. The gentle flat surface above the gorge is the top surface of the west dipping fault block.

 

Little Blitzen Gorge was carved into the fault block by glaciers during the Pleistocene ice ages, beginning about 1.6 million years ago. The glaciers left the steep sided U-shaped valley that can be seen in the photo. Nearby Kiger and Big Indian Gorges were formed in a similar manner. 

 

The Steens basalt is well exposed in the seep walls of Little Blitzen Gorge. The layers in the basalt were formed by numerous basalt flows. The flows dip gently west, following the west tilt of the Steens Mountain fault block.

Note

Steens Mountain lies within the Steens Mountain Cooperative Management and Protection Area which includes a number of private and public lands. For more information contact the Bureau of Land Management, Burns District.

 

External Websites

Bureau of Land Management: Steens Mountain

GORP:  Steens Mountain National Back Country Byway

Oregon Department of Geology and Mineral Industries:  Places to See—Kiger Gorge

USGS: Geologic Field-Trip Guide to Steens Mountain Loop Road

Wikipedia: Basin and Range Province,   Steens Mountain,   Columbia River Basalt Group

 

 

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This photograph from the crest of Steen Mountain looks down the eastern escarpment of the mountain toward the Alvard Desert, which is obscured by smoke from fires south of Steens Mountain. The west-dipping basalt is well exposed along the escarpment (white lines). During the ice ages, glaciers cut cirques in a number of places along the eastern scarp.    

 

The basalt flows at Steen Mountain are referred to as the Steens Basalt and are over 3000’ thick. The eruptive center is thought to be in the area of the Pueblo Mountains, a few miles south of Steens Mountain. The eruptions began about 17 million years ago and lasted for 1.5 million years. These eruptions occurred at the same time as the other flood basalts of the Columbia River Basalt Province.

 

Many of the basalts at Steens basalts have large flat white crystals of plagioclase feldspar that help distinguish this basalt from other Columbia River basalts.

 

The faulting that uplifted and tilted the Steens block to the west began about nine million years ago and is still in progress. The fault zone is about 30 miles long, with maximum uplift of about 7000’. The Steens block is broken into a number of large segments by other faults, as shown in the diagram. 

 

As the Steens block was uplifted the Alvord Valley dropped down and began collecting sediments from the eroding fault scarp. These sediments are now about 2000’ thick. Lakes developed in the Alvord Desert during Pleistocene ice ages, and then disappeared about 10,000 years ago.

Eastern Escarpment

Steens Mountain