Geology 101: Leslie Gulch


In addition to driving on stupidly dangerous goat paths, WC spent the weekend in the deeply dissected volcanic rock of the Mahogany Mountain Caldera, in the southeastern corner of Oregon, near the Idaho state line..

Leslie Gulch, Owhyee Mountains, Oregon

Leslie Gulch, Owhyee Mountains, Oregon

Leslie Gulch is actually inside that 15.5 million year old caldera, which is some 15 kilometers wide and 20 kilometers long. The caldera was created when it ejected some 280 cubic kilometers of rhyolite ash. Some of that ash – ranging in size from dust-sized particles to chunks of rock – fell back into the caldera. Ash from those eruptions and from the later Three Fingers Caldera filled up much of the huge Mahogany Mountain caldera. Fifteen million years of subsequent erosion has created a spectacular region.

Geology is messy. But the result of the eruptions and erosion is fascinating, and scenic even to a non-geologist. If you think of the ash deposits as a messy kind of gigantic layer cake, where the cake was mixed and baked by 4-5 cooks, all  using different recipes, you have some idea what to expect when erosion exposes “slices” of the cake. Here’s an exposure of several hundred meters of rhyolite ash.

Rhyolite ash outcrop, about 300 meters deep, Leslie Gulch, Owhyee Mountains, Oregon

Rhyolite ash outcrop, about 300 meters deep, Leslie Gulch, Owhyee Mountains, Oregon

Here’s what geologists discovered as they worked their way through that exposure.

United States Geological Survey, Report 89-77, Plate 2, by Dean B. Vander Meulen

United States Geological Survey, Report 89-77, Plate 2, by Dean B. Vander Meulen

When ash is still very hot when it lands, it welds together, forming a fairly soft rock. Rain and snowmelt erode that soft rock differentially, creating “hoodoo” formations.

Weld tuff hoodoos, Leslie Gulch, Owhyee Mountains, Oregon

Welded tuff hoodoos, Leslie Gulch, Owhyee Mountains, Oregon

All of that ash was laid down in an active volcano crater. Molten rock – basalt – was forced up through cracks in the welded tuff. When the tuff eroded away, the much harder basalt remained. Vertical veins of basalt are called dikes. Here’s a classic dike exposed in Juniper Canyon, off Leslie Gulch.

Basalt dike, Juniper Gulch, Owhyee Mountains, Oregon

Basalt dike, Juniper Gulch, Owhyee Mountains, Oregon

Basalt cooling underground congeals into columns, often vertical columns but occasionally the horizontal columns see in this dike. Some dikes carry molten rock all the way to the surface, where it emerges as lava.

When lava did appear, its hardness as a surface rock greatly slows erosion.

Basalt cap rock on rim of Mahogany Mountain caldera; gray-colored ash flow in foreground

Basalt cap rock on rim of Mahogany Mountain caldera; gray-colored ash flow in foreground

Note the darker rock on the top of the rimrock; that’s basalt. You can see the results.

But Leslie Gulch is very impressive even if you don’t care about geology. It’s spectacular to see. It’s well worth the drive. Facilities are pretty limited. There’s a campground near the westerly end of the road, close to the boat ramp at Lake Owhyee. There are camping areas on nearby Succor Creek. And the area gets blast furnace hot during high summer. But the scenery is hard to beat.

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One thought on “Geology 101: Leslie Gulch

  1. Thanks for the geology lesson. Geology of the western landscape is one of those areas I always seem I wish to know more about but gets pushed to the back burner.

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