The Twin River formation of Arnold and Hannibal is redefined to include a mappable sequence of predominantly argillaceous, sedimentary rocks that are exposed in the northern Olympic Peninsula, Washington. These rocks are typically exposed on Deep Creek, but well exposed sections of the formation are also present on the Lyre River and along the Strait of Juan de Fuca between the mouth of the East Twin River and the mouth of Murdock Creek. The Twin River formation overlies the Lyre formation and is overlain conformably by the Clallam formation as used by Weaver. In the south limb of the Clallam syncline, where it is best exposed, the Twin River formation has maximum thickness of 17,500 feet.

Three mappable units are recognized in the formation: a lower member consisting of thin-bedded sandstone and siltstone; a middle member of massive siltstone that grades westward into bedded siltstone and sandstone; and an upper member that is composed chiefly of massive mudstone. Lenses of conglomerate occur in places in the formation but only one conglomerate lentil in the middle member is thick enough to be mapped at a scale of 1:48,000. The Twin River formation contains marine mollusks and Foraminifera that indicate an age range of late Eocene to early Miocene.