The Tualatin basin is surrounded by the Portland Hills and Tualatin Mountains to the north and east, the Chehalem Mountains to the south, and the Coast Range to the west. Cooper and Bull Mountains lie within the southeastern part of the valley. Elevations range from approximately 1,100 ft in the highlands to an average of 175 ft on the valley floor.
Approximately 14,000’ marine and continental sediments were deposited here between 50 and 20 million years ago. These are on top of an oceanic basaltic basement laid down about 55 million years ago in the Tualatin basin As much as 1000 ft of Columbia River Basalt (about 15 million years old) and up to 1500 ft of overlying continental sediments blanket the marine sediments. A thin capping of 125,000 year old Missoula flood silt covers the Tualatin Valley to elevations of approximately 250 ft.
Cooper and Bull Mountains break up the lowlands of the basin. Faulting is more prevalent in this area, especially on the east side of these two mountains. In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch or trench. A horst is the raised fault block bounded by normal faults or graben. A graben is the result of a block of land being downthrown producing a valley with a distinct scarp on each side. Graben often occur side-by-side with horsts. An eastern horst extension of Cooper Mountain is buried under Willamette Silt. South of the horst is a complex graben structure bounded on the south side by the northeast-southwest-trending Sherwood fault.
The lowlands of the area wrap around the southern end of Bull and Cooper Mountains to join a subbasin west of Cooper Mountain. Two east-west-trending, fault-controlled Columbia River Basalt highlands extend west from Cooper Mountain sometimes reaching the surface and dividing lowlands at the west- into two separate depressions. South of the Sherwood fault there is a subbasin that follows the topographic low of the Tualatin River floodplain to the mouth of the river.
This area is crisscrossed by a wide range of trails that will allow you to see the various plant communities supported by the variety of soils created by this active geologic history. To log this cache, email the cache owner with the following:
1) Describe the differences in the topography you can see to the south and to the east from the posted coordinates. Which direction shows the uplifting most clearly.
2) Go to the waypoint included on the cache page. The informational sign tells you something about the soils in this area. Compare the plant growth here to that you walked through to reach this point. How do you think the soil effects the differences you see.