The Green River region lies about 30 miles southeast of Seattle on a flat bench of glacial gravel. Its perimeters are older sedimentary mountains to the west and north, with the Cedar River forming a snaking northern boundary; towering Cascade foothills to the east; and the lush Enumclaw floodplain to the south. Black Diamond lies in the heart of the Green River region.

Millions of years ago, an array of geologic occurrences...