From Tolkien Gateway
Grindwall was a small hythe (landing) on the north bank of the Withywindle. It was outside of the High Hay and so was well watched and protected by a grind or fence extending into the water. On rising ground behind the hythe was the village of Breredon.
In one hobbit-poem Tom Bombadil left his cockle-boat there on his way to the Marish. After visiting Farmer Maggot, he vanished and the boat stayed there for three days. Shire-hobbits say that otters came to take it back along the Withywindle and with other animals pulled it up to Tom's creek; however they forgot the oars behind, and "Long they lay at Grindwall hythe for Tom to come and find them".