While looking at a topo map of North America, I noticed two interesting features:

(1) The entire west coast is really mountainous. The Sacramento Valley, Willamette Valley, and Columbia Gorge are among the few areas at sea level between the coast and central plains

(2) What is this smile-shaped basin in southern Idaho?

That's the Snake River Plain. Coincidentally (I think), tectonics dragged the eastern half of the plain over the Yellowstone Hotspot between 10 and 5 MYA