Virgin of the Rocks, Louvre: Geologic Detail

The rocks that extend from below the basal contact line near the Virgin’s head down to the foreground are sandstone, like those at the top of the grotto.  The texture and rounded weathering pattern of the sandstone are the same below the basal contact as they are at the top of the grotto. In the foreground, the sandstone has not been heavily weathered and has therefore retained its highly defined horizontally layered (or bedded) structure.  The diabase sill at the center of the formation is harder and less prone to erosion, hence its sharp edges and vertical relief.
The jagged rocks rising from a blue-gray mist in the background are remnants of erosional processes that stripped away the overlying softer rock and left the harder rock remaining.  These formations have been subtly yet accurately depicted, consistent with Leonardo’s unwavering commitment to geological realism.