Columbia River Basalt capping older Miocene and Oligocene rocks
Lots of basalt covers much of eastern Washington south of the great bend in the Columbia River. The basalt is part of a large igneous province, one of the largest out pourings magma in the world. What is under all that basalt?
On the north, one can get peaks along the thin parts of the Columbia River Basalt Group along the northern margin of the massive flows near the Columbia River. This includes a favorite spot HERE and an obscure but easy to visit spot on Highway 2 HERE. Both show essentially igneous rocks that extend to the north into the highlands across the north part of the state.
On the west, a drive down along the Columbia River near Brewster will encounter similar rocks and closer towards Chelan one can see the a highly metamorphosed and very ancient accreted terrain that is an extension of the North Cascades (a future post).
To the east the basalts thin south of Spokane and one can see the ancient former edge of North America. These rocks also show up deep in the Snake River canyon down stream of Lewiston, on the peak of Steptoe Butte (steptoe-butte) rising as a lone summit above the Palouse which is otherwise underlain by basalt and thick wind deposited silts, and hard to get to isolated outcrop on an island in a lake surrounded by basalt cliffs (bonnie-lake-precambrian-schist and more-notes-on-bonnie-lake-steptoe).
To the south one has to head down into Oregon and into the deep canyon lands of the John Day region. In a way the rocks here are the most interesting as they preserve what the area was like before all the basalt flooded over the landscape. These rocks are full of plant and animal fossils from between 25 million and 15 million years ago before all the eruptions.
These fossil sites have been a treasure of information about the Miocene Oligocene and a lot of what we know of the period is from the Oregon sediments preserved under the Miocene age basalt of the Columbia River Basalt Group at the John Day Fossil Beds.
The National Monument provided a nice labeled picture of the same view I took.
Of course the next question is Exactly where do the rocks seen around the west, north, east and south edges of the Columbia River Basalt Group meet under the basalt?