The Mancos B Formation is a complex of intimately interbedded and interlaminated claystone, siltstone, and fine sandstone formed in a marine environment about 100 miles east of the Emery shoreline. The lower Mancos B was deposited as east-west trending, northward prograding fore slope sets built on a sub-horizontal ocean floor; sandy facies are found as far north as Douglas Creek anticline but rapidly change to a shaly facies farther north. Deposition of the upper Mancos B began with an influx of sandy material, probably from the west, that filled an erosional (?) topographic low in the North Douglas Creek area and spread southward; some clastics continued to enter the area from the south or southwest. The formation is "sandiest" at Douglas Creek anticline, where it is about 500 feet thick, and "shaliest" in the southeast, where it can be over 1000 feet thick.