Hammer Stone/Biscuit Discoidal.

Round Hammer Stone

Hammer stones are often heavily damaged or even broken in pieces.They will be found where flint is being mined or sites where large flakes of flint are worked into points. Discoidals are usually recovered from large dwelling sites or ceremonial centers where larger numbers of people would gather. 

These stones could also be Anvil Stones, used by Northern Tribes like the Mandan and Hidasta of the Knife River area in North Dakota to make pottery, Click here to see how