Did you notice that the hills before you are a different color than those behind you? Why is this? These hills are the eroded remains of a volcanic sill, a body of dark volcanic rock that was shot up from a plume of magma deep in the earth. However, it did not erupt. A rock layer above the magma prevented it from continuing to the surface. The magma, under incredible pressure, instead found a weakness that made moving horizontally easier than breaking through the cap above it; thus it injected itself between two layers of light-colored limestone. This event happened over 10 million years ago when the rock around you was still deep underground. Erosion has since worn away those rocks, exposing the volcanic sill that was once hidden. Soils weathered from the sill are more acidic than the surrounding limestone soils, producing more favorable growing conditions for some plants, and less favorable for others. Look for a change in plant types as you pass beyond the dark sill.