Leonardo's Geology
Figure 3
1. Spherically weathered sandstone
2. Contact surface
3. Bedded sandstone
4. Columnar joints
5. Bedded sandstone (dipping away from viewer)
6. Basal contact
7. Diabase sill (not exactly 90 degrees)
8. Erosional remnants
From top to bottom: The rounded, weathered mounds (spherical weathering) of
horizontally layered (bedded) sandstone, a sedimentary rock, form the top of the grotto. The
column of rock above the Virgin's head is diabase, an igneous rock. When it was deposited on
the sandstone, the diabase was a molten liquid. As it cooled, it formed a layer of rock called a
sill. It also shrank in volume. The resulting contraction caused the rock to crack perpendicular
to the sandstone, forming columnar joints. The columnar joints are not perfectly vertical, but
inclined slightly. This implies that the sandstone dips a few degrees away from the observer,
an impression that is borne out by close study of the layers.