Virgin of the Rocks, Louvre 1483-83, Leonardo da Vinci

The Virgin of the Rocks in the Louvre is a geologic tour de force because of the subtlety with which Leonardo represents a complicated geological formation. The grotto is made of weathered sandstone dissected by a layer of harder rock.  At the top of the grotto are rounded (spherically weathered) mounds of sandstone, a sedimentary rock.  Above the Virgin’s head is a rock that juts out in vertical relief.  This is diabase, an igneous rock that was injected as a molten liquid and spread over the sandstone, forming a band (or a sill) several feet high.  The rock contracted as it cooled, forming vertical (columnar) joints.  Directly above the Virgin’s head is a horizontal crack in the rocks called a basal or bottom contact.  This is the seam between the diabase about and the sandstone below.  The column of diabase extends upward until it meets another horizontal contact surface and the rock formation changes to sandstone at the top of the grotto.