Sourdough Mountain is a 6,111-foot peak in North Cascades National Park in northern Washington, rising in a notably rugged part of the Cascade Range shaped by steep ridges, deep glacial valleys, tectonic activity, terrane accretion about 50 million years ago, and repeated Pleistocene glaciation. Near its summit stands the Sourdough Mountain Lookout, built in 1933 by the Civilian Conservation Corps, reflecting the importance of remote mountaintop stations in forest fire detection; it was added to the National Register of Historic Places in 1989. The mountain’s marine west coast climate brings heavy precipitation, especially wet winter snow, moderated temperatures, and significant avalanche danger.