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GSOC Field Trip to Bull Run Watershed
by Janet Rasmussen
Saturday, June 25:
It was one of the hottest days of the summer, but you would not have known it in the shady, breezy, high elevation Bull Run Watershed east of Portland. Nearly 30 people from the Geological Society of the Oregon Country (GSOC) met in East Portland to board the bus for Bull Run. A couple of City water employees led the trip with Bev Vogt, GSOC Secretary.
It was an exciting time to be a geologist back in the 1970's: our understanding of tectonic plate movement was new, as was magnetic pole reversal, and its usefulness in determining the relative age of rocks. The Hanford nuclear facility was being built on top of Columbia River basalts, which were still not well understood. Bev Vogt was a graduate student in Geology at PSU, and worked with Marvin Beeson to map and study the basalts of the Bull Run Watershed, which provides Portland's drinking water. Throughout the field trip, she shared with us the discoveries she made as she tramped into the steep ravines of the watershed, occasionally crossing paths with cougars.
As we drove into the watershed from above, we saw a black bear run down the road ahead of us. It disappeared into the woods before I could get out my camera. At Bull Run Lake, we walked up near the edge where some old cabins remain. Portland began using water from Bull Run back in 1895. The water derives from precipitation, not from nearby Mt. Hood, as some have thought. The forest is made up of Douglas Fir, hemlock, and cedar. Blue lupine and penstemon, white beargrass, and pink rhododendrons were blooming.
Bull Run Lake with Mt. Hood in the background
One of three log cabins, which are in poor repair and not being used at this time.
We had a picnic lunch, and Bev and Bart laid out a display of samples of rock from the many strata making up the bedrock geology. The various flows of basalt are chemically unique, and some of the differences were visible to the naked eye.
Our driver, Gail, did an exceptional job of maneuvering the long bus on the narrow, curving road. At one point, she began to make a tricky turnaround and riders were heard to mutter with alarm. Bev very tactfully asked Gail to let the passengers disembark "as some had concerns for their safety, and please don't take it personally..." As we filed off, Gail cheerfully remarked to us, "It'll be easier without the extra weight, don't take that personally!"
"Bart" Bartels broke rock so that we could collect a few samples.
One of the interesting discoveries that Bev made was that the ancient Columbia River didn't always flow where it does now. At some time, it flowed right where Mt. Hood is now, and emptied into the ocean south of its present outlet.
The basalts of Bull Run come from two Formations, the Wanapum, represented here by two Members (6 flows of the Frenchman Springs Member, and one of the Priest Rapids Member; on top of the Grande Ronde (14 to 15.8 million years ago), made up of three mappable units. Between these two Formations is the Vantage horizon, a layer of charred organic material and soil that developed in the perhaps hundreds of thousands of years between the basalt flows. We saw this horizon clearly near the dam on Reservoir 1. The Vantage horizon is found over parts of Washington and Oregon, and varies in thickness from just an inch or two to several feet.
The dam on Reservoir 1
The Vantage horizon is easy to spot, a paleosol ("old soil") that appears as a thin line just below the overhanging foliage.
Below the dam on Reservoir 2 is the chlorination facility. The water isn't filtered, but disinfected in case of pathological bacteria that might be present. Below Reservoir 2 is Kaiser Park, an area once maintained by Water Bureau workers. There are several lovely old fountains and a water wheel apparatus dated 1894. At our last stop, we viewed some of the core samples from a well drilled 600 feet into Kaiser Park. The Vantage horizon was black siltstone, about two feet thick here. Beneath that were several other separate flows, with a possible fault zone exhibited at about 340 feet.
Bev is explaining the thrust fault she discovered within the Bull Run Watershed area.
Just a couple months ago, another fountain was discovered hidden in brush across the road from the others. Decorated with quartzite cobbles and bits of broken brown and green glass, it bears the name "Laline" written in glass below the waterline.
