S, 2 people at the flint show asked me the same thing so I'm ready for the Q.
There is absolutely ZERO topsoil / sedimentary soil on top of the fire rock.
Go back to the top of this posting and look carefully at the area where my finger is pointing ( not the shadow )...There are modern roots growing right in the original ash. Flakes & worked pieces are found from the roots right down to the yellow lowest original soils.
Where the new road and straight in digging is, appears to be very level and the terrain above & below are sloped. SOME downward movement must have
happened and that would explain the mixture of all ages of points.
Whole mound only about 80 feet in diameter.