We addressed three questions relevant to patterns of mountain goat abundance in Washington's North Cascades: 1) What are forages used by mountain goats during summer? 2) Is canopy cover of mountain goat forage species predicted by geological substrates? and 3) Are indices of nutritional quality and digestibility of two mountain goat forage species predicted by geological characteristics? These questions were motivated by observations that historical abundance of mountain goats in Washington, accounting for habitats generally documented as suitable for them, was greater over some geological substrates than others. Mountain goats ate primarily sedges, secondarily rushes, and made surprisingly little use of grasses. Mountain goats ate a wide variety of forbs, with none showing overwhelming use. Despite their abundance in many landscapes near mountain goat escape terrain, Vaccinium spp. were rarely consumed, and other shrubs in Ericaceae were avoided entirely. Geological substrate explained only a small proportion of variability in mountain goat forage availability. Categorized by geological origin, sedges had higher canopy cover when over sedimentary and shale substrates than when over plutonic substrates. Categorized by geochemistry, sedges had higher canopy cover when over sedimentary rocks than over potassium-feldspar substrates. Sodium-rich substrates generally supported less vegetation than other substrates across all forage categories. Neither nutrients nor digestibility of the two focal species were predicted by geological type. Our study suggests that geological substrates in the North Cascades vary slightly in their production of forage plants valued by mountain goats, but do not affect the nutritional quality of two key forage plants.