The East Bay Hills are an elongated block of Precambrian rocks and is composed of two rock types: primarily volcanic deposits of the Fourchu Group (ash and lava interleaved with marine sediments), but also granites. They are elongated northeast-southwest in accordance with the same prevailing trend of faults which dominates the geological structures of the North Bras d'Or Uplands. The southeastern margins are also faulted, in this case bringing the resistant Precambrian strata against late Carboniferous sandstones and siltstones. On the northeastern side the contact is with easily eroded Carboniferous Windsor Group strata, which were deposited directly against the blocks when they stood as islands in the Carboniferous Sea.