The post-Knox unconformity provides the principal trapping conditions for underground gas storage at Unionport in Randolph County. Erosion remnants of the porous Knox Dolomite protrude upward into the overlying impermeable Black River Limestone. Although the unconformity has long been recognized in Indiana, data for detailed mapping of its surface are lacking except at Unionport. Here about 50 wells drilled to the Knox outline a surface of steep-sided, ovate erosion remnants of limited lateral extent with vertical relief of about 150 feet. No drainage pattern is evident. The unconformity is postulated to have been developed in an arid environment with wind as the primary erosion agent. The erosion-induced irregularity of the post-Knox unconformity is only slightly reflected on the top of the Trenton Limestone. The composite thickness from the top of the Trenton to the top of the Knox ranges from 400 to 530 feet at the Unionport gas-storage project. Petroleum has been produced in Indiana, Ohio, Kentucky, and southern Ontario from erosion remnants associated with the post-Knox unconformity. We feel that petroleum deposits under similar geologic conditions exist in Indiana and that the crestal area of the Cincinnati Arch provides the most favorable general area for exploration.