This is a near-textbook demonstration of limestone cave formation by subsurface running water.
Initially, flow along a bedding plane dissolves a lens-shaped (in cross section) channel: a phreatic tube, completely filled by water, as it's below the local water table.
If the water table subsequently drops (possibly because a cave network has developed! ) the cave may cease to be completely water-filled (becoming vadose) and further solutional erosion will be restricted to the floor. Hence, a vertical slot is incised, producing a cave with a T-shaped cross-section, as here. Ignore the loose rock (roof collapse?) at the junction of the 'T', or mouseover for an overlay of the approximate cross-section.
From memory, I'd estimate the vertical section is ~1.75 m high by ~0.75 m wide, whilst the original 'lens' at the top is probably ~0.3 m high by ~1.75 m wide.
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