Threatening Rock was a huge monolith (150x30x100 feet) detached from a vertical scarp of Cliffhouse Sandstone on the northeast side of Chaco Canyon, N. Mex. Systematic measurements of its movement, begun in 1935, indicate a total translational movement of about 6 feet away from the cliff before the mass toppled in 1941. There was a geometric increase in average rate of movement with time with greater movement during the 7-month killing frost period of winter. Extrapolation of log plots of movement and time, assuming uniform climatic and other conditions, indicates movement of the rock began 2,500 years before collapse. Final collapse was caused by center of gravity moving past the support beneath the overhang of the front face.