Ice Age Plunge Pool
[312.0 MM 220 Interstate 90 East]
When we arrive in Ritzville we can take a side trip to Palouse Falls (or take this trip on another day). This is another large cataract and plunge pool formed by the ice age floods.
|"Palouse Falls drops 185 feet, and occurs at the intersection of northeasterly and southeasterly trending joint systems (Fryxell and Cook, 1964). The Palouse River was apparently diverted across this divide area from its pre-flood route along Washtucna Coulee. The present river occupies a narrow slot-like canyon excavated to a maximum depth of 400 feet below the scabland surface of the Palouse-Snake divide." [Field Trip 2]|
|"The falls seem to appear out of nowhere since we have been traveling over the rolling Palouse Hills. The canyon is very deep and winds downstream to the Snake River. I was impressed by the castle-like appearance of the eroded basalt columns just above and to the left of the falls. To the far left is a flood-caused cave similar to the Lenore Caves. The park overlook has a grassy area with picnic tables and a paved path to the edge of the canyon." [Draggoo]
The floodwaters that carved this channel left not only the falls but a large cave (left) plucked from the basalt and an intriguing "castle" of eroded columns (right) near the edge. They can be seen in relation to the falls in the top left in the photo above.
In the photo below you can see how the channel winds down and around as the Palouse River makes its way to the Snake River. In the distance at the top of the channel are loess hills that have been streamlined as they were eroded by the floodwaters.
When water flows over a falls it erodes the area below, called plunge pools. Through a process called abrasion and plucking, water currents can erode the base of the falls and cause the front edge of the falls to collapse into the plunge pool. See diagram at right. Click on diagram or here for a
larger image. In this way a river flowing over a falls can "move" the edge of the falls upstream and deposit broken rock downstream.
Image Copyright © EWU Press
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- A waterfall with a single, sheer drop. Usually with a large volume of water flowing over the falls.
- The deepest part of a river or bay.
- Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
- Plunge Pool
- When water flows over a falls it erodes the area below leaving a hollow spot. The material that was eroded is deposited downstream.
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