Of these mountains, the Cascade range stretches approximately 1,200km from northern California to southern British Columbia.  It runs more or less parallel to the Pacific Coast at an estimated 150 - 250km inland. The western slopes of the Cascades are subject to high snow loads in the winter and comparatively wet summers. These mountains shield the interior regions inland to the Rocky Mountains from the prevailing weather of the Pacific Ocean.

They give rise to the dry belt areas of British Columbia, Washington and Oregon.Throughout the coast ranges of mountains a diverse geological history that encompassed mountain uplift, volcanism, and glacial advances and recessions provided evolutionary selection forces that acted on floras originating from the south, the west (amphipacific), the north (circumpolar), and the east.  These forces gave rise to the floristic diversity that is found now.