Wow that's huge, don't know much about ancient stone tools, but I bet that's something a museum would love to have - thus you are very lucky to have found it. Maybe even a banner worthy find!
That sure was a big piece of Kaolin they used to knock that one out.... Wow... Heck of a piece to have in any collection...
I was out in that area once, got permission for a field and a creek, found a few chucks, about 5,000 flakes in the field and also one small nodule of cobden, no artifacts, depending on how technical one might be. Cuz flakes are really artifacts too, but anyway...
Awesome piece, thanks for sharing...