Geological Information

Since about 35 million years ago, oceanic crust from the Pacific Ocean has been subducting under the continental margin, which has formed the current volcanoes as well as a number of igneous intrusions composed of diorite and gabbro. (DNR) The current uplift of the Cascade Range began around 8 million years ago.

The Methow Valley contains the thickest stratigraphic section of Cretaceous sedimentary rocks in the state. The Cretaceous consists of 29,000 to 60,000 feet of marine black shales, sandstones, conglomerate, and arkose. The top of the section is marked by red-purple sedimentary rocks. Protolith ages of adjacent terranes are Permian to Jurassic. Some of these terranes, such as in the Chelan mountains, contain limestone, basalt, tonalite-diorite, gabbro, and ultramafic pods.

The northeastern Cascades are known for mineral deposits of various geologic ages. Triassic calc-silicate gneiss, biotite schist, and marble were hosts to the state’s largest copper producer, the Holden mine in Chelan County. It produced 207 million pounds of copper along with zinc, silver, and gold from 1938 to 1957. There is a porphyry copper deposit in a Cretaceous stock near Mazama.These intrusives are the plutonic counterparts of volcanic rocks in the southern Cascades.

Former mining camps are scattered throughout the northwestern Cascades. Copper, silver, and gold were the main metals recovered. Breccia pipes associated with Oligocene to Miocene shallow plutons contain not only copper but also molybdenum and tungsten. These pipes are favored by collectors of quartz and pyrite crystals. The Spruce claim in the Snoqualmie batholith in King County is a world-class mineral locality. This batholith also contains a large porphyry copper deposit.