The Wallace Formation and its stratigraphic equivalents of the latePrecambrian Belt Supergroup in Montana and Idaho were deposited in an elongate northwest-trending basin. The axis of maximum accumulation coincides approximately with the center line of the basin, and the thickness decreases symmetrically away from it; the maximum thickness is 16,500 feet, near Libby, Mont. Limestone and dolomite are the principal rocks on the eastern side of the basin and are characterized by the presence of oolites, stromatolites, and mud-chip breccia. Clastic rocks with minor carbonate constituents make up most of the section in the center and on the western side of the basin. Graded bedding, crossbedding, channel structures, and ripple marks are the typical sedimentary structures in the clastic section. Mud cracks are present throughout the area but are most common in the lower part of the section.