Cross-valley profiles from the west slope of the Olympic Peninsula, Washington, are used to investigate the relative effects of fluvial and glacial erosion on valley formation. Unlike most ranges where glaciers and rivers sequentially occupied the same valleys, neighboring valleys in the Olympic Mountains developed in similar lithologies but were subject to different degrees of glaciation, allowing comparison of the net effect of glacial and fluvial processes integrated over many glacial cycles. Upslope drainage area was used to normalize comparisons of valley width, ridge-crest-to-valley-bottom relief, and valley cross-sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley-spanning transects. Valley width, relief, and cross-sectional areas are similar for glaciated, partly glaciated, and unglaciated (fluvial) valleys with drainage areas of <10 km2, but diverge downslope. Glaciated valleys draining >50 km2 reach two to four times the cross-sectional area and have to 500 m greater relief than comparable fluvial valleys; partly glaciated valleys have intermediate dimensions. At distances of >5 km from valley heads, the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in fluvial valleys. The finding of strong differences in the net result of valley excavation by fluvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to Pleistocene glaciation.