Lower Allegheny detrital rocks thicken to the north and west. The Vanport limestone thins to the west-southwest, northwest, and north by interfingering with marine shales and to the northeast by gradation into chert and flint clay. The area of thickest limestone was an area of tectonic stability. The wedge complex was an area of subsidence and differential filling. East-west changes across the northern wedge complex are more rapid. The tectonic pattern during lower Allegheny time is one of stability along a west-northwest trending belt and differential subsidence to the north and southwest.