A geological interpretation of Fishmonger’s Swallet is presented and places the cave within the context of previously published geological data from the area North of Bristol. Geological observations were made and hand specimen rock samples collected. The cave is located close to the outcrop boundary between the Penarth Group shales and the Clifton Down Limestone on the West flank of the Coalpit Heath Syncline and close to the Ridgeway Fault. The host limestone has a sparry and micritic composition and also contains two thin white claystone seams, likely formed from degraded tuffs. The limestone strata dip moderately to the East-South-East and are cut by numerous wrench faults. Near the bottom of the cave a North-East plunging anticlinal axis is seen and the oldest strata exposed in the cave are ascribed to the topmost beds of the Lower Cromhall Sandstone. Rocks within the cave are commonly altered and cut by calcite veins and minor fractures and these likely formed the focus for a mixed speleogenesis history with both upwelling phreatic waters and later vadose solution.