The Torrey Sandstone crops out near the top of the sea cliffs and at many other locations in the reserve, for instance on the Fleming Trail, the cliffs by the main Reserve road, and in Canyon of the Swifts. The “type section” where the rock was first described is on the Torrey Pines grade which runs through the reserve. On the geologic map, it is dark green. It is mostly quartz with some feldspar, usually white but often stained light brown by iron oxide from the rocks above. The rock was deposited as a sandbar. The loose sand was cemented later by calcite from water flowing through the sand. Few fossils are found in the sandstone because not many creatures live in a sand bar.
From the Rim Trail in Canyon of the Swifts, we can see about 100 feet of Torrey Sandstone in the cliff across the canyon. The small caves in the rock are called “wind caves” although wind plays only a small role in making them. Water running over the cliff dissolves the cement between the quartz grains, and water and wind carry the grains away. Uneven cementing of the rock leads to the first shallow holes, then, because the holes are shaded, the backs of the holes stay wet, more cement is dissolved, and the holes deepen. A close-up of one cave shows the shaded back.
A concretion is seen in the back of the shaded cave. The concretions are caused by deposition of calcite and iron oxide cements from solutions running through the sandstone. Rainwater dissolves the cements from the sandstone and the rocks above it during wet times and deposits them during dry times. Deposits grow on earlier deposits so the concretions often start at a point and grow in nearly spherical layers. If one starts along a fossil twig, for instance, it can grow into a concretion that looks like a pipe.
The sandstone in the canyon also shows dewatering cracks, the vertical white streaks that carry the brown layer bands upward. The original sand dune was deposited in water so it was saturated. Before the sand was cemented into rock, something shook the sand so that the grain structure in a layer collapsed and water in the layer carried the load of sand above it. The high-pressure water squirted up through any crack, carrying loose sand with it. Later cementing preserved these cracks as we see them.
Next formation: Lindavista