Succession of interbedded grey, white and brown sandstones, and dark-grey, partly carbonaceous mudstones and siltstones. Individual sandstone beds are more than 10 m thick, but stacked beds can amount up to 80 m. The sands are mainly fine- to medium-grained, with occasional gravelly intercalations. Sandstone beds can have sharp tops and bottoms, or grade into mudstones. Mudstone-dominated intervals with thin silt- and fine-grained sandstone intercalations can be up to 100 m thick. The formation consists of stacked coarsening-upward cycles, generally starting with a basal marine interval. (Cameron 1993).
In the Dutch reference section, some 1 - 4 m thick argillaceous limestone beds and tuff layers are intercalated. Sandstone beds are up to 17 m thick here.
Sheet-delta systems with brief marine incursions (Collinson 1988). Sandstones: delta-front deposits with mouth bars, and distributary-channel fills. Fines: pro-delta turbidites, or settled from suspension. The Millstone Grit Formation reflects a depositional regime, intermediate between the basin facies of the Epen Formation and the delta-plain facies of the Klaverbank Formation. As a result of the north-to-south regressive development of the lower Pennsylvanian - upper Mississippian (Namurian-Westphalian) in the Silverpit Basin area, the two formations are partial age equivalents.
Placed at the base of the lowermost sandstone interval which is more than 10 m thick.
Placed below the Klaverbank Formation at the base of the sandstone bed immediately underlying the first definite coal seam, or the base of that coal seam where such a sandstone is not developed (Cameron 1993).
