PWAX photo from:
The uppermost stone appears to have been enhanced so as to suggest a protruding nuchal as well as the marginal scutes, still observable after weathering for an unknown length of time...
The placement of the "wedge" further suggests an artistic choice to create this probable/possible zoomorphic and rather testudinate petroform.
Was the encircled portion of the lower stone "pecked" as well?
Was this the original position of the "wedge?"
Was it a spot for a clam shell of Tobacco?