Traveling Rocks & Gravel Bar Deposits EarthCache
Traveling Rocks & Gravel Bar Deposits
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
The locations of gravel river bars are determined by the geometry of the river and the flow through it. Point bars form on the inside of meander bends in meandering rivers because the shallow flow and low shear stresses there reduce the amount of material that can be carried there. The excess material falls out of transport and forms the bar. As with the entire region, the underlying bedrock in the Tualatin Watershed is Columbia River Basalt. Exposures of Columbia River Basalt can be seen along the watershed boundaries on the Coast Range, the Portland West Hills, and the Chehalem Mountains, as well as along the final 3.4 miles of the Tualatin River near West Linn. Overlying the Columbia River Basalt are flood deposits from two major flood events. The low-elevation, bowl-shaped Tualatin Basin was able to capture and retain many rich, flood-deposited soils. The first layer consists of unconsolidated deposits of gravels and sands known as the Troutdale Formation, deposited by ancient rivers from the east. The Bretz Floods left rich alluvial (water deposited) soils on top of the Troutdale Formation after the waters receded. In the center of the Tualatin Basin, near Hillsboro, 1,500 feet of sediment bury the Columbia River Basalt. The Bretz flood brought many rocks trapped in ice bergs. Some of these rocks are erratic and basalt. There presence are still found locally. #1. What can you see here that gives you an idea of what made the curves in the river. #2. How many rocks are on display at this site. #3. What type of rocks are they? #4. What is in the round metal disk embedded in the cement at this site? This earth cache is put here with the permission of City Of Tualatin And Metro.
Additional Hints
(No hints available.)