The Carrock was a stony eyot in the upper reaches of the River Anduin, to the north of the Old Ford a few miles west of Beorn's farmstead.
History
During their journey to the Lonely Mountain (Erebor), Thorin and Company were housed there overnight by the Eagles before they went to see Beorn.
In Chapter 7 of The Hobbit, Gandalf states that the steps from the base of the rock to the flat top were made by Beorn and that "Carrock" is Beorn's name for it. This is somewhat of a linguistical joke on Tolkien's part, since carr in Anglo-Saxon means "rock". It could also be related to the Welsh carreg, meaning "stone."
References
- The Hobbit: Out Of The Frying-Pan Into The Fire
- The Hobbit: Queer Lodgings
- Geiriadur Welsh-English On-line Dictionary: http://www.geiriadur.net/index.php?page=ateb&term=carreg&direction=we&type=all&whichpart=exact