Paleontology and geology
Exposures of Ordovician slates occur in the northeastern-most corner of Washington. These rocks are metamorphosed shales formed from muds that were deposited in a deep ocean off the west coast of North America. The presence of these deep-water deposits tells us that sea levels rose during this time, and the shoreline migrated eastward into what is now Montana. Graptolites and conodonts are abundant in these rocks; trilobites can also be found, but they are rare.