Gully regularly covered by mad prophet’s rock carving (10)

'rock carving' is the definition.
(I know that rock carving can be written as petroglyph)

'gully regularly covered by mad prophet's' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'covered by' means one lot of letters goes inside another.
'mad' indicates anagramming the letters.
The alternating letters of 'gully' are 'gly'.
'prophet' with letters rearranged gives 'petroph'.
'gly' placed into 'petroph' is 'PETROGLYPH'.