Basalt quarry

The massive pit in front of you is a former gravel quarry that was excavated by during the construction of the Caldecott tunnel in the 1930s. While the hole itself is not a natural feature, it exposes the inner workings of Round Top volcano. The wall opposite you is made up of basalt lava flows that buried the vent of the volcano, forming a sort of cap. Fossil clams and snails have been found in the tuff deposits of this quarry, indicating the occurrence of a freshwater lake that filled the caldera between eruptions. Note the color and texture of the basalt: the rock weathers to a light, yellowish color over time (keep this observation in mind at future stops). Due to the motion of the bounding faults, the rocks at Sibley have been transported to the northwest, and they have been tilted to the east. This means that Round Top was originally located south of where you now stand (relative to land to the east), and structures you see here are laying on their side. If the volcano were to be put right side up again—and your current position is at the railing looking east over the quarry—you would be laying on your back!

Before you depart this spot, be sure to notice the giant stone labyrinth at the bottom of the quarry. On another occasion, when you have the time, head down there to explore!