When I left the GSA meeting in Portland, I decided to take a different route back, one that would normally be shorter, except for several sidetrips that I took. After figuring out how to get away from downtown Portland despite faulty Google Maps directions, I immediately headed east on I-84, which goes up the Columbia River. My first goal: stop at Multnomah Falls (Google Maps location), one of my favorite places. To that end, I got off the freeway and took the old road, the Historic Columbia River Highway, which goes by several falls.
The first view of the falls, from the parking lot area, made it seem as though it was coming out of the low-hanging clouds. Would I be able to see the entire falls?
The upper and lower Multnomah Falls, and the footbridge, from a little ways in. The bridge - Benson Bridge - was built in 1914 by Simon Benson, an early emmigrant to Oregon.
Multnomah Falls - the second highest year-round waterfall in the U.S. - flows over part of the Columbia River Basalt Group (CRBG), specifically five flows of the Grande Ronde Basalt, which is the lower part of the Yakima Subgroup of the CRBG (Mangan et al, 1986). Many of the online links I found say that six flows are exposed in the cliff face of the falls; from what I can tell from an original source, six flows of the Grande Ronde Basalt are exposed from the Columbia River to the top of the falls, with pillow lavas being at or just below the very top of the upper falls (Norman and Roloff. 2004). Norman and Roloff's strat section makes it look like there are five flows in the cliff faces of the uppper and lower Multnomah Falls. Anyway, check out their drawing and see what you think!
These are the lower falls, with Benson Bridge above and the upper falls above and behind the bridge.
It's a moderately easy hike on this trail, through wonderful rainforest - see the ferns! - to the bridge.
At the bridge, this sign tells part of the story of a huge rock that came off the face behind the upper falls in 1995. The rest of the story is told here. The huge rock mass that fell off the cliff shattered when it hit the upper plunge pool, resulting in minor injuries to some people on the bridge. When I was there in late 2000, five years after the rockfall, the falls didn't look the way I remembered - from visits decades earlier and from photographs.
The entire upper falls. Can you tell where the rock fall came from? (It's shown on the sign.)
Well, I'm thinking that the rock fell from the large diagonal fracture area just above the curvilinear, nearly horizontal fracture cutting across the entire lower part of the photo. I base this conclusion partly from memory, and partly from a July, 1995, photo on Wikimedia that shows a rock mass instead of a diagonally fractured area. Also, compare the full resolution version of the Wikimedia photo to Ron Schott's gigapan of Multnomah Falls, from August 6, 2009.
And one more thing before we look higher: the rock in the center foreground of this photo is not "the" rock that fell in 1995. For one thing, it's not large enough to be the bus-sized rock. For another thing, the mass that fell shattered into pieces upon hitting the ground. And besides that, this rock is present in old photos including the Wikimedia photo of July, 1995.
This tan area near the top of the falls shows some interesting radiating to irregular columnar jointing in the basalt, just below some of the pillow lavas, which can be seen in the upper part of this photo (enlarge to see the pillows).
And finally, here is a close-up view of the uppermost part of the upper falls, which was almost hidden in the low-hanging clouds. I've walked the trail to the top - once, I think - back in 1993. I highly recommend the trail just for the immersion into the rain forest. And there's a great view looking straight down the falls.
Ooh, neat water near the bottom of the upper falls!
Another shot of the wonderful rainforest, this time on the way back toward the parking lot. Anyone who visits Multnomah Falls should consider eating in the historic Multnomah Falls Lodge, which was built in 1925. This time, I just stopped at an outdoor coffee and snack stand and got a hot chai tea to go.
Mangan, M.T., Wright, T.L., Swanson, D.A., and Byerly, G.R., 1986, Regional correlation of Grande Ronde Basalt flows, Columbia River Basalt Group, Washington, Oregon, and Idaho: Geol. Soc. America Bulletin, v. 97, p. 1300-1318. [also see this Geoscience World link]
Norman, D.K., and Roloff, J.M., 2004, A Self-Guided Tour of the Geology of the Columbia River Gorge—Portland Airport to Skamania Lodge, Stevenson, Washington: Washington Division of Geology and Earth Resources, Open File Report 2004-7.
UPDATE 4Jun2010: For a view of the basaltic rock types at Multnomah Falls see this post at Geology Blues.