The Olympic Mountains are primarily sedimentary with some volcanic basalt at lower elevations (see USDA data). The mountains are high enough to block cloud movement from the Pacific Ocean. The western side of the mountains is in one of the areas of greatest precipitation in the continental United States, with an annual rainfall of about 90 in. at the coast to over 200 in at higher elevations; the northeast side, in the rain shadow, is one of the driest areas on the West Coast with 15 in. at Sequim and 10 in. just north of Sequim (see rainfall map).
There are at least 8 endemic plants in the Olympic Mountains, several are shown below. The best alpine plants occur in the dryer alpine zones on the east and north sides of the Olympics. The best known of these is the Mt Townsend and Buckhorn/Marmot Pass areas of the east side. These are accessible by trail only. The areas from Mt Angles to Deer Park are accessible by road from the Port Angles area.