Multnomah Falls, a two-tiered feature falling 620 feet, cascades over Grande Ronde Basalt, a basalt formation in the Columbia River Basalt Group.
Columbia River Basalt, a collection of fissure lava flows, covered parts of Idaho, Oregon and Washington. The flows are divided into five formations - Grande Ronde, Imaha, Picture Gorge Basalts, Saddle Mountains and Wanapum. The flows are Early Miocene, dated between 5.5 and 17 million-years-old, and erupted from fissures near the ID-WA state line. The group consists of ~300 flood basalt flows, each from 10 to 100+ feet in thickness, with a volume of at least 700 cubic miles - the largest lava flows on Earth. The flows are 16,000-feet thick in the Pasco Basin, and in the Columbia River Gorge, 21 flows formed a thickness up to 2,000-feet. [1]
After eruption, the Missoula Floods of the last Ice Age scoured the cliff, leaving the falls hanging high above. This makes Multnomah Falls an ideal place in the Columbia River Gorge National Scenic Area to study flood-exposed geology. Five basalt flows are visible in the cliff face.
Water continuously erodes the Columbia River Gorge walls as evidenced by curving cliffs forming amphitheater-like basins. Water flows into cliff-wall cracks, freezes and thaws, wedging rocks tumbling to the ground. Falling rocks are common at Multnomah Falls. In 1995, a bus-sized piece of basalt fell from behind the waterfall into the pool, and in 2014, a boulder smashed through the Benson Bridge.
You can see three types of basalt at the falls:
(1) Entablature basalt - forms by fast-cooling lava fracturing into irregular patterns and joints.
(2) Pillow basalt - when basalt flows meet water, they solidify almost immediately, forming round "bubbles" of rocks.
(3) Columnar basalt - forms when lava cools slowly and fractures to form 5- or 6-sided crystals.
To complete this Earthcache:
(1) At posted coords, find the upper falls' height, in feet, on the small stone plaque. Include the answer in your email.
(2) Proceed to N 45 34.593, W 122.06.935 - the first hairpin-turn past the Benson Bridge. Look right of the falls and look for the boundaries between the five basalt flows. Estimate the thickness, in feet, of the third (middle) layer of the five.
(3) Estimate the distance, in feet, from the top of the third basalt layer to the top of the falls. (Use the number from the stone plaque to help with scale.).
Email me the answer and please do not post in your log. Please don't post pictures that would reveal the clue or answer either.
[1]: Swanson, D.A., and Wright, T.L., 1981, Guide to Geologic Field Trip Between Lewiston, Idaho, and Kimberly, Oregon, Emphasizing the Columbia River Basalt Group