The Columbia River Gorge

The passage through the Columbia River Gorge by the Lewis and Clark expedition between 28 October and 3 November 1805, was as geologically noteworthy as any seven-day period experienced by the Corps of Volunteers for North Western Discovery. Their roughly sixty-seven-mile transit from The Dalles to the Sandy River bore fresh evidence of gigantic landslides, drowned forests, and disruptive volcanic eruptions.

After a much needed respite from the arduous passage through Celilo Falls and the Long and Short Narrows of The Dalles, the expedition pushed off from Rock Fort on the morning of October 28, 1805, and began their entrance into the Columbia River Gorge.

[28 October 1805:] we proceeded on river inclosed on each Side in high Clifts of about 90 feet of loose dark coloured rocks. —Clark

Those familiar “dark coloured rocks” belong to the Columbia River Basalt Group (CRBG).^[3]See Columbia River Basalts. Most of the lava flows in the Gorge are Grande Ronde Basalts deposited during the peak of Columbia River flood basalt eruptions between 16 and 15.6 million years ago. Although the Gorge presents dramatic exposures of the CRBG, it actually has fewer individual lava flows than the upriver terrain the expedition had just passed through.^[4]Although the CRBG has been estimated to be greater than 13,000 feet thick in the Pasco Basin area, it is only some 4,000 feet thick in the Gorge per Jim E. O’Connor and Scott F. Burns, … Continue reading

Having traveled only about five miles on 28 October, it would be on 29 October 1805, that Clark took note of the dramatic topographical changes (and the increase in botanical diversity) of the Gorge as the expedition entered the present-day area of Rowena Gap:

we proceeded on, the mountains are high on each Side, containing Scattering pine white oake & under groth, hill Sides Steep and rockey.

The Columbia River Gorge was not formed by the Glacial Lake Missoula floods (as is sometimes erroneously claimed in travel guide literature^[5]http://www.frommers.com/destinations/oregons-columbia-gorge/ 627619#sthash.G1MYQMWQ.dpbs. Accessed 16 September 2015.), although the superb view of the lower cliff sections of the Columbia Hills anticline at Rowena Gap were certainly accentuated and reshaped by flood scouring. The consolidated rocks that compose the Gorge were formed by a complex interplay of CRBG flood basalt deposition and basin subsidence, along with contemporaneous folding and faulting. At the same time the CRBG flows were occurring in the middle and late stages of the Miocene Epoch,^[6]The Miocene Epoch was 23 to 5.3 million years ago; the CRBG flows commenced about 16.7 million years ago. the region was experiencing intense compression stresses.^[7]This compression was originally thought to have been largely caused by very complex subduction-related stresses between the North American Plate and various tectonic plates off the coast of the … Continue reading Shortly after lava flows were emplaced, this unceasing compression transformed the original, slightly uneven layer-cake arrangement of the flows into mile-scale folds akin to rucks or ripples in a carpet. This geomorphology of ridge-forming narrow anticlines separated by valley-forming broad synclines can be observed throughout the Columbia Plateau.^[8]The structural subprovince exemplified by N70°E-trending anticlinal ridges and synclinal valleys is called the Yakima Fold Belt. The width of the anticlines and synclines are quite asymmetrical, … Continue reading

The combination of contemporaneous basalt deposition and deformation are responsible for the warped geological structure that can be observed in certain places in the Gorge, such as the Columbia Hills at Rowena Gap. However, this region would have remained an expansive, nondescript plain with a handful of low-relief ridges if not for the regional-scale uplift caused by the rising High Cascades that brought the Gorge into being. Beginning some three million years ago,^[9]The author has come across different age estimates for the beginning of the regional uplift so a median age was chosen for the main text of the article. Two million years is estimated on page 476 in … Continue reading the central axis of the High Cascades commenced rising over 3,000 feet upward,^[10]O’Connor and Burns, “Cataclysms and Controversy,” 238. exposing the CRBG to the relentless incision and entrenchment of the Columbia River, which created the magnificent Gorge that William Clark described as having mountains “high on each Side.”

Dutifully mapping the Columbia River on 29 October 1805, Clark noted:

passed three large rocks in The river the middle rock is large long and has Several Squar vaults on it. we call this rockey Island the Sepulchar.

Lewis, who apparently only kept a journal on the return journey through the Gorge, reported on their brief foray exploring the island:

[15 April 1806:] we halted a few minutes at the sepulchre rock, and examined the deposits of the ded at that place . . . there were thirteen sepulchres on this rock which stands near the center of the river and has a surface of about 2 acres above high-water mark.

This nearly bare, roughly flat-topped mass of Wanapum Basalt^[11]The bedrock geology of Memaloose Island appears not to have been officially published by either the Oregon Department of Geology and Mineral Industries (DOGAMI) or the United States Geological Survey … Continue reading is known as Memaloose Island, and its stripped surface indicates it bore the full brunt of Missoula flood scouring. It became one of the many islands in the Columbia river used as a Native American burial ground, which Lewis and Clark appear to have treated with due respect (i.e., there is no indication in the journals that any “deposits of the ded” were removed by the Corps of Discovery). Unfortunately, this was not an example emulated by later settlers, who performed their own scouring of remains and artifacts until the extant native remains were removed prior to the partial inundation of the island by the Bonneville Dam impoundment.