“…Inferences that the Belt Basin extended far to the south of their present remnant were based solely on tenuous facies relations and thickness trends (Reynolds, 1984). On the Grasshopper thrust plate, Missoula Group (upper part of the Belt Supergroup) rocks are widespread and more than 7 km thick (Ruppel and Lopez, 1984), but, east of the Grasshopper decollement, outcropping Belt strata, which are also Missoula Group, are restricted to a small area in the core of an anticline near Argenta, Mont. ( fig.…”