Unaweep Canyon (Uncompaghre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large-scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high-resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The dataset is further complemented with a high-resolution electrical resistivity survey. Integrated with other geophysical and geological data, the results unambiguously demonstrate an overdeepened Precambrian basement with pronounced transverse U-shape and corroborate the hypothesis of a pre-Quaternary glacial origin. Our data constitute the first detailed and high-resolution image of a buried pre-Quaternary glacial valley in North America, and thus have far-reaching implications for our understanding of global ice houses as well as the tectonic conditions enabling preservation of such systems.