This rock unit formed when thick layers of sand built up on the ocean floor, about 41 to 47 million years ago.
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/geology/rocks/tenino-sandstone/tenino-sandstone.md
The grains of sand that make up the Tenino Sandstone are very small, measuring about the size of a grain of sugar.