The teleseismic P wave residuals of three underground nuclear explosions (Rulison, Rio Blanco, Gasbuggy) and three Denver earthquakes were examined as a function of azimuth. The location and origin times of these six sources are known to a high accuracy, and they are all located in or near the Colorado Rocky Mountains. The data covered the azimuth range from approximately 50 ¿ west of north to 160 ¿ east of north. The P residuals were corrected for the average receiver residuals. It was found that the travel times in the sector from N40 ¿W to N are clearly delayed by about 1 s compared to the sector from N to N40 ¿E. This observation can be interpreted to be due to a termination of the pronounced western U.S. low-velocity layer, which coincides approximately with the eastern edge of the Rocky Mountains.