Pressure Ridge, California

Pressure ridges form where lateral motions on a curving fault force rocks into a smaller space
pushing them upward. (more below
 
 
 
 
 
 
 
 
Faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree. When a bulge on one side of the fault is carried against a bulge on the other side, the excess material is pushed upward. (And where the opposite occurs, the ground is depressed in a sag basin.) The Hector Mine earthquake of October 1999 created this small "mole track" pressure ridge in the Mojave Desert. Pressure ridges occur in all sizes: along the San Andreas fault, its major bends coincide with mountain ranges like the Santa Cruz, San Emigdio and San Bernardino Mountains
 
 

Sag Basin, California

Sag basins occur along the San Andreas and other transcurrent (strike-slip) faults. They're the counterpart of pressure ridges. (more below

  
 
 
Strike-slip faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree (see the three types of fault). When a concavity on one side of the fault is carried against another on the other side, the ground between sags in a depression or basin. (And where the opposite occurs, the ground rises in a pressure ridge.) Where the ground surface of the sag basin falls below the water table, a sag pond appears. This example is from the San Andreas fault just south of the Carrizo Plain near Taft, California. The two sag ponds lie in a larger rift, a linear valley. Sag basins can be quite large; the San Francisco Bay is an
example.
 
 

Stream Offset, California

Stream offsets are the counterpart to shutter ridges, a sign of lateral movement on strike-slip faults like the San Andreas fault. (more below)

 

 

 
This stream offset is on the San Andreas fault in Carrizo Plain National Monument. The stream is named Wallace Creek after geologist Robert Wallace, who documented many of the remarkable fault-related features here. The great 1857 earthquake is estimated to have moved the ground sideways about 10 meters here. So earlier earthquakes clearly helped produce this offset. The left bank of the stream, with the dirt road on it, can be considered a shutter ridge. Compare this picture to the shutter ridge picture, which is exactly analogous. Stream offsets are rarely this dramatic, but a line of them is still easy to detect on aerial photos of the San Andreas fault system.
 
Fault Scarp, California
 
Motion on a fault may raise one side above the other and create a scarp. This fault scarp formed in the 1872 Owens Valley earthquake. (more below)

 

Fault scarps are short-lived features in geologic terms, enduring no more than a few millennia at best; they are one of the purest tectonic landforms. But the movements that raise scarps leave a large area of land on one side of the fault higher than the other side, a persistent elevation difference that erosion can obscure but never erase. As fault displacement is repeated thousands of times over millions of years, larger escarpments and whole mountain ranges—like the high Sierra Nevada range beyond—can arise.
 

Escarpment, Oregon

Escarpments are long, large breaks in the land that separate high and low country. They may result from erosion or from fault activity. (more below)
 
 
The escarpment called Abert Rim, in south-central Oregon, is the site of a normal fault where the land in the foreground dropped by several kilometers relative to the plateau behind, one large earthquake at a time. At this point the escarpment is more than 700 meters high. The thick bed of rock at the top is the Steen Basalt, a series of flood basalt flows erupted about 16 million years ago.

Abert Rim is part of the Basin and Range province, where normal faulting due to extension of the crust has created hundreds of ranges, each flanked by basins many of which contain dry lake beds or playas. Abert Rim may be North America's finest example of an escarpment, but the area has several other contenders. The world's premier escarpments, though, are probably in Africa's Great Rift Valley.