Sunday, October 21, 2012
The Sierra Nevada Underground: Into the Black Chasm
The state has such a complex geologic history involving granite intrusions, volcanic eruptions and earthquakes that it is easy to forget that for vast stretches of geologic time, California was stunningly boring passive continental margin. In Paleozoic time, the region was a shallow sea, slowly collecting mud and lime deposits in a tropical environment. In some cases, the tropical islands and shorelines were elsewhere, but the rocks were transported to California on the giant conveyor belt of the Pacific and Farallon tectonic plates. As the Farallon plate was subducted beneath the continental margin, the limestone sequences were scraped off and added to the metamorphic sequences of the western Cordillera.
The limestone was baked and transformed into marble, and a great deal of marble was preserved in a tract of crust (or terrane) in the western Sierra Nevada Metamorphic Belt called the Calaveras Complex. The Calaveras hosts many of the caverns found in the state; there are hundreds of them. Unless I lost count somewhere along the line, six caverns in the Sierra Nevada have been developed for public tours: Crystal Cave in Sequoia National Park, Boyden Cave in Kings Canyon, and four caves in the Mother Lode: Moaning Cave, Mercer Cavern, California Caverns, and today's destination, Black Chasm Cavern.
The problem is that many Sierra caves were discovered by miners in the 1850s, and they became well-known and well-visited. It was not at all unusual for the cave owners to encourage their visitors to take a stalactite as a memento of their visit, and the caves were stripped of their decorations (speleothems would be the proper term for the stalactites and other features of caves). The early explorers often used torches for light, and the soot left its mark on the remaining speleothems.
When the owners of Black Chasm decided to open the cave for tourism, they constructed a walkway that clings to the wall of the Black Chasm and passes into the rooms beyond. The contrast between the first room of the cave and the last room is striking. Many of the stalactites and other speleothems are broken off, and the remaining ones have a patina of orange-red mud that comes from seepage of iron oxides from soils above, but also from soot and dirt from the early visitors. The room is still pretty, but not exceptional to anyone who has visited other caverns.
The final room of the tour is exceptional because of one particular kind of speleothem: helictites. Helictites can be characterized as stalactites that refuse to recognize the law of gravity, or by some descriptions, stalactites on acid. They have grown in random directions, perhaps because the water that formed them was driven as much by capillary action as it was by gravity.
More information about the cave can be found here. Tell them Geotripper sent you!