Mesozoic sedimentary rocks overridden by thrust plates in the disturbed belt of northwestern Montana have been metamorphosed by burial beneath these plates. Bentonite in the Cretaceous section has been converted to potash bentonite by this metamorphism, allowing radiometric dating of the emplacement of the thrust plates by the K-Ar method. Ages determined thus far range from 72 to 56 m.y. B.P. The oldest ages coincide with estimates for the beginning of thrusting from stratigraphic and structural evidence. Field evidence does not allow a reliable estimate for the end of thrusting. Thus, the younger ages, which indicate completion of thrusting at the end of Paleocene time, represent additional data of interest to the structural history of the disturbed belt.