Steens Mountain

For a Google Earth .kmz file to explore Steens Mountain, scroll to the very bottom of the page.

The Steens are a remote fault block mountain that’s found in the southeastern portion of the state of Oregon, south of the city of Burns and close to the tiny town of Frenchglen. If you’re in the mood for some really remote camping and backpacking, the Steens is the place to go. It is also located in an area commonly referred to as the “Great Basin”. The area is coined as such because the drainages of the area do not flow out to the ocean but instead water tends to collect in lakes and wetlands. A great example of this would be the Great Salt Lake in Utah. If you’re a geologist, the area is a treasuretrove of different geologic processes well recorded in the morphology of the mountains. There are three basic events that we are going to talk about in regards to the features that can be seen here; 1) the deposition of the Steens Basalts, 2) the formation of normal faults uplifting the Steens Mountain, and 3) glaciation carving out the valleys found along the edges of the Steens.

So, first and foremost, the rocks that have been exposed during the uplift of the Steens Mountain are mainly basaltic rock that is part of a larger group of basalt flows called the Columbia River Basalt Flows (CRB’s for those in the know). These flood basalts cover a very large area in the Pacific Northwest. The CRB’s were deposited between 17 and 5 million years ago from large fissures that opened along the Oregon/Idaho/Washington border (https://volcanoes.usgs.gov/observatories/cvo/cvo_columbia_river_basalt.html). These fissures would have looked like large cracks in the ground, oriented N-S, with lava being emitted along their length. The fissures were 3 miles long in some cases and the lava emitted flowed out over much of the border between Oregon and Washington. The lava made its as far as the Pacific Ocean and even flowed south to Newport, Oregon. These flows were extremely voluminous and are some of the best preserved and exposed flood basalts on Earth (which is part of the reason I always tell my geology classes, if you’re going to learn to identify one rock here in Oregon, that rock should be basalt). There are over 40 different basalt flows that are associated with the CRB’s and the Steens Basalts are just one of those basalt flows (Figure 2). After the basalts were deposited then came the normal faulting that is associated with Basin and Range topography.

We have Basin and Range topography here in Oregon in the very southeastern portion of the state, but this topography stretches far beyond Oregon and can be found in California, Idaho, Utah, Nevada, Arizona, and New Mexico (basically the American Southwest). Basin and Range topography forms due to normal faulting, which is the type of faulting that forms due to extensional forces called tension. But before we talk about that let’s discuss the definition of a fault. A fault is a break in the crust where rock has actually displaced from its original position. A really well-known example that we all have heard about, particularly because of a movie with the Rock, is the San Andreas Fault. However, the San Andreas Fault is not moving in the same fashion as the faulting that created the Steens Mountain. Instead of moving horizontally like the San Andreas Fault, the Steens fault zone moves in a more vertical motion. As the crust near the Steens Mountain was pulled apart, blocks of the crust lowered to accommodate for tensional stress, exposing older rock at the surface and creating a near vertical fault surface, referred to as a fault scarp. This type of fault movement is referred to as a normal fault because the hanging wall, or the rock block above the fault surface, has dropped relative to the footwall, or the rock block below the fault surface (Figure 3).

The Steens Mountain fault block began rising and tilting west approximately 9.3 million years ago and the offset of the fault movement here has exceeded more than 1 mile (Evans and Geisler, 2001). This is a very short geologic time, believe it or not, which is why the relief of the east flank is so steep. In fact, the area is still active with the last movement on the fault occurring approximately 1,500 to 2,000 years ago (2001).

The last geologic process of note in the area are the very clearly glacially formed valleys that can be seen on the western side of mountains. These valleys include Big Indian Gorge, Little Blitzen Gorge, and Kiger Gorge to name a few. Glaciation of the area began around 1.6 million years ago, during the Pleistocene epoch (2001). The formation of glaciers in the region was due to a much cooler climate that allowed for snow and ice to accumulate at lower elevations than today. Glaciers are incredibly good at eroding the surfaces they travel over. They can cut deep into bedrock and smooth out the rock that it moves over, moving a tremendous amount of rock as the glacier flows downhill. There are a couple different textbook features that can be found in the Steens Mountain including u-shaped valleys, cirques and tarns, hanging valleys, and moraines. The most obvious glacial features are the u-shaped valleys. These valleys extend from the top of the Steens Mountain mainly to the west and the north. These form as the glacier travels down from its starting point and erode deep into the bedrock. U-shaped valleys differ from river valleys because they have with steep sides and a flat bottom (Figure 4). It is very obvious by looking at Little Blitzen Gorge, for instance, that is has been glacially formed because the valley edges are not obscured by vegetation and show a perfect example of that characteristic U-shape.

The next feature created in the Steens is one that marks the location of the very top of the glacier is a formation called a cirque. A cirque is a bowl-shaped feature cut directly into rock where the glacier originates. Sometimes this bowl can be filled with water and the resulting lake is referred to as a tarn. There are several tarns that can be found just below the Steens ridge in both the Big Indian Gorge and Little Blitzen Gorge. Lastly, at the base of these same two gorges, or the mouths of the gorges, there are piles of sediments that have been bulldozed by the glaciers from inside the valley to its current location. These loose piles of rock are referred to as moraines and are composed of a glacial rock deposit known as till. When you hike in this area, even before you get inside of the gorges, it is pretty easy to tell glaciers have come through the area because there are huge boulder littering the ground that are much too large and not well-rounded enough to have been deposited by water. Now, if you do hike out in this area, please be careful, I have come across an angry mountain lion in Big Indian Gorge and it was the most terrifying hike I have ever taken.