Linda,
 
I recently worked on a cemetery from the same time period.  We were just looking
 
to define the boundaries of the cemetery by identifying the grave shafts.  We
only had to go about 1- 1.5 feet deep to distinguish the shafts.  However, we
did excavate deep enough to actually encounter one of the graves, which we found
 
at about 4.5 feet deep.  It sounds just as likely to me that grave diggers could
 
have stopped at the gravel layer. I do not know if there was a standard grave
depth, I always heard most were at 6 feet.  I hope this helps.
 
 
M. Jay Stottman
Kentucky Archaeological Survey
University of Kentucky