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Rhythmites from Lake Lewis flooding (Burlingame Canyon)

View on map:46.022315°N 118.593543°W

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When the ice dam (that formed Lake Missoula) broke, the Wallula Gap proved to be a choke point for the water which backed up to form Lake Lewis. At first, the flow deposited a sand layer (larger sediment) as it gradually slowed. As the water backed up, the water laid deposits with flow patterns opposite of those in the flooding. Large fine sediment followed as the lake formed. Once the water passed through the gap, the deposition was in the direction the water flowed again through the gap. After the flooding silt was laid down. The next flood would develop a similar deposit. You can see the various deposits accounting for over 40 massive floods. Within each deposit is a typical structure of backflooding, flow out of the lake, and flooding again (see reference).

The canyon here was formed by humans when a canal overflowed exposing the layers in  1926.

On the 12th rhythmite from the top is a layer of Mount St. Helens ash deposit 16,000 years old.  

Note the vertical clastic dikes that cut the rhythmites (cause still debated). 

Access to the canyon is restricted. 

Description


Rhythmite

A rhythmite consists of layers of sediment or sedimentary rock which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, by shorter term processes such as tides, or by longer term processes such as periodic floods.

References

  • http://en.wikipedia.org/wiki/Rhythmite
  • https://floodexplorer.org/items/show/63#:~:text=Deep%20in%20the%20heart%20of,in%20the%20multiple%20flood%20hypothesis. 
  • https://www.youtube.com/watch?v=b9PJ7LPVGSA
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