Stones like that were also called abraders.....in flint knapping you have to dull the edge of the piece your are working to create "platforms" from which to work flakes. The abrading stone was actually created by "sawing" the edge of the arrowhead or knife across it dulling the edge to make knapping easier, thus forming grooves in the softer stone....most here are made of some type of coarse stone...many times sand stone.
Atlantis