This picture shows a long stretch of shingle beach, which is showing signs of erosion. As the waves wash over the beach, the pebbles are moved around, knocking into each other, breaking into small pieces and becoming more rounded, until eventually they turn into fine shingle / sand.
Relevant image keywords:
background, beach, coastline, full frame, gravel, nature, no people, outdoors, pebble background, pebble beach, pebbles, shingle, stones
