Chapel Porth is situated on the North Cornwall coast just South of St Agnes. It is owned and managed by the National Trust. There is a small car park and cafe. The beach is a small sandy cove which at low tide extends to a wide strip beyond the cliffs. When the tide is out you can walk south along the beach to nearby Porthtowan. Alternatively, the South West coast path extends along the cliff top in either direction
You are standing in one of the many caves scattered along this stretch of coastline formed by coastal erosion. If you look directly above you'll see the roof has collapsed. The boulders lying in the sand in front of you are probably what's left of it. The cave extends behind you and to the right (look at the footprints in the sand) and re-emerges 20 metres or so further down the beach.
