Four seams of coal, of which three are workable (No. 1, No. 2 and No. 4), occur in the Upper Cretaceous Nanaimo Group, Comox Formation interbedded predominantly with sandstone and minor conglomerate. The coal bearing strata rest unconformably on Upper Triassic Vancouver Group, Karmutsen Formation volcanic rocks and some of the lower seams are pinched out against basement topographic highs. The coal is often associated with shale and may contain thin interbeds of carbonaceous shale or bone coal. The lowest seam tends to contain more of the above than seams higher in the succession and in places becomes extremely shaley. The roofs and floors of the seams are predominantly sandstone.

The seams strike generally northwest and dip approximately 6 degrees northeast. The area contains a number of northwest to south- east trending normal faults which subdivide the area into (tilted) fault blocks. A second set of more east-west trending faults are also present.

The coal is high volatile bituminous "A" in rank. The lowest (stratigraphically) seam, the No. 4 seam, varies from 0.9 to 2.1 metres in thickness and generally consists of clean coal. This seam has been the most extensively mined in the Comox (Cumberland) area. Mines No. 1, No. 2, No. 3, No. 4, No. 6 and No. 7, with the No. 4 mine being the most productive. The next most productive seam, the No. 2 seam, occurs 36.6 metres above the No. 4 seam and has been mined in mines No. 5 and No. 8. The seam is approximately 1.1 metres thick. The uppermost seam, the No. 1 seam, varies from 0.8 metres to 2.1 metres thick and has been mined at mines No. 5 and No. 6, and to a small extent at the No. 2 slope.

The coal in the Cumberland area contains 0.45 to 4.30 per cent moisture, 6.80 to 27.22 per cent ash, 28.09 to 37.70 per cent volatile matter, 42.44 to 55.95 per cent fixed carbon, 0.20 to 3.92 per cent sulphur and has a value of 10,148 to 14,246 BTU per pound (as received except for G.C.V. which is on a dry basis).

The Cumberland area has produced approximately 16 million tonnes of coal (1969). In-situ reserves (1973, B.C. Hydro) are estimated to be 3,466,000 tonnes probable and 279,710,000 tonnes possible, making a total of 283,176,000 tonnes. This is considered high by A.R.C. James (1974).