The entrance to the Empire Cave, as seen on the left, partially hidden behind the overgrown thicket of bushes, and protected by a cement wall. The view is looking up Cave Gulch, with the small stream on the left and Kresge and Porter colleges up the hill to the right. The University has tried to cover the entrance to the cave several times with cement and steel structures, only to have it dynamited each time by irate cave enthusiasts. Several other caves occur in the area, with entrances across the creek. Caves are common worldwide, wherever karst terranes are found. As discussed at the sinkole site, caves form when acidic groundwater disolves the CaCO3 marble. This process can occur most easily below the water table because the CO2 is needed to make carbonic acid in the groundwater:
The image on the right looks down the entrance to the Empire Cave. Most of the caves along Cave Gulch were probably formed as acidic waters percolated along faults and fractures, as can be seen from the north-south orientation of the Empire Cave passage way. Many of the canyons on campus and joints in the quarry walls are oriented in this direction as well. These directional features are controlled by a persvasive north-south fracture system in the marble.
A number of characteristic cave features, such as stalactites and stalagmites, are found in the Empire Cave. These formation grow by the deposition of CaCO3 from waters that percolate through the rock. Presumably, as the waters reach the air of the cave, dissovled CO2 escapes, decreasing the ability of the water to hold the calcium carbonate in solution. This process enables CaCO3 to precipitate as the water drips down the stalactites, which grow down from the ceiling, and onto the stalagmites, which grow up from the floor.
The last rock type found on campus is represented here by this granitic boulder photographed in the Cave Gulch creek. A closer look reveals an igneous rock with interconnecting grains of quartz, plagioclase feldspar, amphibole, and biotite. This rock is a member of the Ben Lomand Quartz Diorite. It is exposed in Cave Gulch and in many roadcuts on Empire Grade. In many places, heavy rains have decomposed the amphibole and biotite, leaving a sandy soil composed of quartz and feldspar. the Ben Lomand Quartz Diorite is part of a number of intrusions that occur within the Salinian Block and shown on this map in red. The granitic rocks of the Salinian Block are similar in composition and to those of the Sierras and are believed by some to be a southerly extension of the Sierras that has been transported northward along the San Andreas Fault. Both the Sierran batholith and the plutons of the Salinian Block were probably represent the deep roots of volcanoes formed along the over-riding plate of a subduction zone. Some magmas reached the surface and formed volcanoes like Mt. St. Helens or Mt. Ranier, whereas some magmas were somehow trapped beneath the surface and cooled slowly to form the plutonic rocks we observe today.
