The picture shows a fresh water mussel out for a jaunt. The U shaped trail is about 8-10 cms wide and the creature making it is about 13 cm long.
I find the shells of this animal on local river banks and lake shores but have not seen one this size alive in its habitat. It looks it uses its foot to move through the sediment in search of food. Picture taken in Kentucky.
Thanks to my friend Herb for sending this image along.