Final answer:
Southeastern Washington's bedrock is mostly composed of basalt, an igneous rock, rather than sandstone, which is a sedimentary rock present in different formations.
Step-by-step explanation:
The bedrock of Southeastern Washington is predominantly composed of basalt, which is an igneous rock. Basalt is a common extrusive igneous rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. By contrast, sandstone is a type of sedimentary rock that consists of sand-sized minerals or rock grains. While sandstone is present in other geological formations, it is not the primary composition of bedrock in Southeastern Washington, where basalt is much more abundant due to the region's volcanic history.