Is this dig-intoable?

TSRM suggest that the structure of light enables you to visually perceive whether a thing is dig-intoable or not.  (Cf. TSRM, 1981, p. 261).  But, do you visually perceive this?  This could be a solid matrix of rock or it could be soft dirt or it could be a thin layer of dirt over concrete.  Can you tell by looking?

Or, run this objection like the others.  Get two boxes.  Fill one with concrete and cover this by a thin layer of dirt and fill the other with dirt.  You cannot visually perceive the dig-intoability of the one versus the other.