High-altitude thermal-infrared images, taken at various times of the day over the Front Range and adjacent plains in Colorado, display many linear and circular topographic features. These features are enhanced by thermal shading effects and commonly are better displayed than in photographs. Comparison with geologic maps of a control area in the Front Range shows that most lineaments drawn from the thermal images represent geologic features, principally faults. A previously unobserved circular feature at the Central City–Idaho Springs mineral district bounds most of the area of gold mineralization; it is a topographic delineation of the most intensively altered ground in the district and may reflect extremely subtle structure related to the presence of an unexposed intrusive body. Lineaments in the plains probably mark subtle structural trends related to the Denver basin and to folds not otherwise expressed at the surface.