At Ash River, coal occurs in the Upper Cretaceous Comox, Pender and Extension formations of the Nanaimo Group. The older Comox Formation contains eight seams, of which seam No. 1 (0.76 to 2.10 metres), seam No. 2 (1.06 metres to 1.20 metres) and seam No. 4 (0.90 to 2.10 metres) are of economic interest and occur over a stratigraphic interval of 30.5 metres. The coals are extremely dirty containing in some cases up to 75 per cent shaly material. The seams are discontinuous and tend to "shale out" throughout the property.

The Douglas and Newcastle seams of the younger Pender Formation and the Wellington seam (refer to 092GSW026) of the Extension Formation are also present in the area. They are of poor quality, with as much as 65 to 80 per cent shale interbedded with the coal. The seams are thin and discontinuous. The coal seams in both formations are interbedded with shale, carbonaceous shale and sandstone.

The Comox Formation is separated from the Pender and Extension formations by the Haslam Formation, Nanaimo Group (85 to 300 metres thick) which consists of sandy shales and shaly sandstones.

The structure in the area is dominated by a northwest trending syncline which is cut off to the northwest by west-northwest trending normal faults which downthrow the Nanaimo Group into contact with the Upper Triassic Karmutsen Formation. The syncline plunges southeast at 3 to 5 degrees and is broad in the south (west limb dips 10 to 15 degrees and the east limb dips 20 to 30 degrees) and tighter and more compressed in the north (west limb dips 10 to 15 degrees while dips on the east limb have increased to 37 to 55 degrees).