You observe a distinct, red sandstone rock body about 50 m thick that is depositionally overlain by a red shale with interbedded gypsum and you follow this boundary in the rock bodies for 1 km until you cross a fault which has displaced the boundary between the two rock bodies. You walk along the fault and see a boundary between a 50 m thick red sandstone and overlying red shale interbedded with gypsum. You conclude .