Olympic's mountainous interior receives over 200 inches of yearly precipitation, much of it falling as snow. Over the years, the snow is compressed into granular firn and eventually into ice. The ice contributes to glaciers that scrape away at the rock below.
If it were not for the erosive powers of water and ice, Olympic's rugged peaks would be miles high, as tall as the worlds highest peaks. All of Earth's builders are kept in check by an opposite force; with Olympic's mountains that is ice.