Science, Tech, Math › Science Gallery of Slickensides Share Flipboard Email Print James St. John/Flickr/CC BY 2.0 Science Geology Types Of Rocks Landforms and Geologic Features Geologic Processes Plate Tectonics Chemistry Biology Physics Astronomy Weather & Climate By Andrew Alden Geology Expert B.A., Earth Sciences, University of New Hampshire Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.S. Geological Survey. our editorial process Andrew Alden Updated January 31, 2019 Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Their formation may involve simple friction, or if the fault surface was once deeply buried, actual growth of oriented mineral grains may respond to the forces on the fault. Slickensides appear to lie between the grinding of shallow rocks that makes fault gouge (and cataclasite) and the deep-seated friction that melts rock into pseudotachylites. Slickensides may be scattered surfaces as small as your hand or, in rare cases, thousands of square meters in extent. The corrugations show the direction of motion along the fault. Unusual minerals may occur given the combinations of fluids and pressures along slickensides. But even familiar rocks, as we will see, take on unusual features too. Slickensides can range in size from tiny, as in a chert specimen, to gigantic. In all cases, you spot them by their telltale glint, and in all cases, they signify shear, the sideways motion of faulting. 01 of 15 On an Outcrop Andrew Alden Slickensides may appear on an outcrop if you face the sun. This is part of the faulted and sheared western face of Point Bonita in Golden Gate National Recreation Area, near San Francisco. 02 of 15 In Limestone Andrew Alden Most rock types can have slickensides. This limestone is also fractured and brecciated by the fault movements that created this slickenside. 03 of 15 Sandstone, Wright's Beach, California Andrew Alden This site is very close to the San Andreas fault, and pervasive fracturing affects this already-jumbled tectonic megabreccia of Franciscan sandstone. 04 of 15 Peridotite, Klamath Mountains, California Andrew Alden Serpentine minerals form easily by alteration of peridotite, especially where faulting admits fluids. These readily form slickensides. 05 of 15 In Serpentinite Andrew Alden Slickensides are very common in serpentinite. These ones are small, but whole outcrops glisten because slickensiding is so pervasive. 06 of 15 In a Serpentinite Outcrop Andrew Alden This larger slickenside is in a serpentinite body at Anderson Reservoir, California, near the Calaveras fault. 07 of 15 In Basalt Andrew Alden Where igneous rocks are tectonically deformed, as at this outcrop in northern San Quentin, California, even basalt can acquire slickensides. 08 of 15 Closeup of Basalt Slickenside Andrew Alden This sample from the previous outcrop displays the aligned mineral grains and polished surface that define a slickenside. 09 of 15 Metabasalt, Isle Royale, Michigan Ben+Sam/Flickr/CC BY-SA 2.0 This exposure from Raspberry Island can be mistaken for glacial striations, but the orientation is wrong. The green color is suggestive of serpentine minerals. 10 of 15 In Chert Andrew Alden In San Francisco's Corona Heights near Peixotto Playground at 15th and Beaver streets is this world-class slickenside in Franciscan chert, exposed by quarrying. 11 of 15 Corona Heights Slickenside, Beaver Street Andrew Alden On the Beaver Street end of this slickenside, the higher surfaces reflect the sky. Slickensides are also called fault mirrors. 12 of 15 Slickenlines Andrew Alden The individual streaks and grooves of a slickenside are called slickenlines. Slickenlines point in the direction of faulting and some features can indicate which side moved which way. 13 of 15 Rock Near a Slickenside Andrew Alden A remnant block from the near side of the fault plane shows the undisturbed form of the chert. 14 of 15 Chert Reflections Andrew Alden The slickenside surface appears hand-polished. Chert is tough enough to preserve this kind of polish against weathering. 15 of 15 Slickenside in the French Alps Centre National de la Recherche Scientifique This large slickenside is on the Vuache fault, at Mandalaz peak in Haute-Savoie.