"sandcastle building"

Now then. When my eldest was a nipper my wife and I began building what we called fairy grottoes on the beach. These used whatever was available in the way of materials; sand, pebbles, slate, discarded packaging, dead fish and crabs, whatever the beach threw up.
They began as modest sized constructions and finished up as huge endeavours with an on-going story to them and the development of every kid and often adult, within reach. Some are circular, others chambered, flat or monumental, whatever the group adds to their making as the idea grows. By their nature they are at the whim of the tide and donated and destroyed by the sea's so we film them as the are being built and erased by the waves.

If that isn't a folk art I don't know what is.