The Sidney Island occurrence is composed of Recent clays of the Capilano Sediments (formerly known as Puyallup Interglacial deposits). Little is known about the Sydney Island clays except that they are similar to the Anvil Island clays (092GNW020) and prior to 1912 they were utilized for the making of stiff-mud common brick (Geological Survey of Canada Memoir 24-E, page 143). It is also reported that a yard was set up in 1907 and produced brick from 1913 to 1918; another company also operated on the island from 1926 to 1929 (Bulletin 30, page 7). No production figures are available.

The clay at the Anvil Island deposits is described as being somewhat sandy and yellowish to bluish-grey in colour and in most places contains fairly abundant pebbles. See Anvil Island for a summary of the firing characteristic of that clay.

James Island, to the immediate west of Sidney Island, also contains similar clay, potentially useful for industrial purposes.