Recently it has been found that casing failures in twenty-two wells lying on the south flank of Buena Vista anticline have been caused by movement on an active thrust fault. Surface evidence of the rate and recency of movement on it is given by its newly made scarp and by the buckling of gas, oil, and water lines at and near its trace. The fault strikes N. 75° W. and dips 25° northward, or into the south flank of the structure. Computations made from measurements on bent pipe lines and collapsed casings show that the overriding block is moving southward at a minimum average rate of inches per year. The fault probably is the result of differential slipping between beds on the north flank of the structure induced by current folding.