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Of course, the devil is very much in the details. The situation at Caerwent is reasonably good for a syphon system: the underlying geology of the "Caerwent Basin" is made up of bands of porous sandstone and limestone with caves. The Troggy cuts into the surrounding high land providing the necessary relief, and we know there was resurgent water leaving the system at the Whirly Pools. It is also plain, however, that the limestone has a number of exits: local Colin Titcombe mapped a number that were open in the 1950s to 1970s* despite the capping in the 1800s. This may be suggestive of a very large series of cavities that could swallow a tide, but might be less conducive to a syphon system which relies on a reasonably air-tight syphon section. Detailed consideration of the tidal inflow, removal rates, resurgent-water tracing speeds, tunnel widths, and possible chamber sizes would be necessary to determine whether the hypothesis is likely.