Known in subsurface and on surface on the Sweetgrass arch and Central Montana uplift in north- and west-central MT where it is assigned as the lower member of the Mowry Shale. Is also one of three named lithofacies--Dynneson, Bow Island, lower Mowry--assigned to the base of the "Dynesson unit." Member name has been applied informally "Bow Island sandstone" to a sandstone that is well developed in the Bow Island gas field of southern AT. Is similar lithologically and stratigraphically to the Dynneson Sandstone Member (named) of the Mowry except that it is less porous and more silty. Has a maximum thickness of 100 ft. Is gray to white, very fine to fine grained, subrounded to well rounded, fair to well-sorted sandstone. Quartz is the dominant mineral, and it makes up more than 90 percent of the rock. Some mica. Silt and clay fill the interstices. Calcareous cement locally. Interbeds of black shale, bentonite, and siltstone common. A few ripple marks and trough cross-bedding in surface outcrops. Cross sections. Isopach map of entire "Dynneson unit." Nomenclature chart. Of late Albian, Early Cretaceous age.

Source: GNU records (USGS DDS-6; Denver GNULEX).