Volcanic rocks of a basaltic type have been encountered at depths ranging from 26m (85') to 36m (118')below sea level (bsl) at the eastern end of Bermuda at Ferry Reach (Hyndman et al 1974), along the Causeway (Golder Associates, 2007) and at Government Quarry (Peckenham, 1981). In the vicinity of Hamilton and on the North Shore in Devonshire, volcanic rock occurs at approximately 45m (150') bsl (author's records) while in Southampton at the Southampton Princess Hotel, volcanic rocks were found at 70m (226') bsl (author's records). A general westward deepening of the volcanic seamount's upper surface is inferred by these data. One explanation for these variations is the survival of the remnants of one or more volcanic calderas. For example, the near-circular body of water known as  Castle Harbour was interpreted by Gees and Medioli (1970), on the basis of their seismic reflection study, as having "bowl-shaped caldera-like sub-bottom" . However, their deduced depths to volcanic rocks, exceeding 60m (200') over large areas, were not at all consistent with much shallower depths recorded in later drilling on the Causeway. A core from a Causeway borehole (photo below) displays a rapid transition from clean white Walsingham (or older) limestone (top left) through a yellowish layer of eroded volcanic debris, which may be a fossil soil, to a heavily veined volcanic rock at approximately 121' below the surface or 113' (34m) below sea level.

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