The coal mined from the Little Ash mine is from the main Wellington Seam which occurs at the base (in the Early Campanian Northfield Member) of the Upper Cretaceous Extension Formation, Nanaimo Group. The seam is on average 2 metres thick, is underlain by sandstone and overlain by sandy shale, sandstone or conglomerate. Variations in thickness and quality of the seams are common. Refer to the Bebans mine (092GSW026) for a description of the Wellington seam.

The beds generally strike northwest to southeast and dip towards the northeast, however, locally at the Little Ash mine, the Wellington seam exhibits some small folds. The mine slope/adit is located at the northwest end of a faulted syncline which is separated from another syncline to the northeast by a narrow anticline. Two main sets of faults trend northwest and north-northeast.