Geology Cafe

Origins of Terraces: Processes That Change The Landscape

Terraces are step-like landscape features that occur in many places across the landscape. They are typically common along coastal areas, around inland lake basins, along stream valleys, and sometimes along fault zones. Terraces reveal information about the history of landscape evolution. Because they are typically naturally flat and elevated surfaces, they are particularly attractive sites for habitation and development.
Marine Terraces
Marine terraces are formed by coastal erosion and reflect the history of the rise and fall of sea level over time. However, marine terraces are only found along coastlines where the land is rising. In areas where the land is sinking they are submerged or buried by younger sediments. On stable coastlines, each successive rise and fall of sea level destroys most of the evidence of previous cycles of sea level change.
Click on thumbnail images for a larger view.
Marine terraces near Davenport in Santa Cruz County. The terraces are forming as Ben Lomond Mountain (to the right) is rising relative to sea level, over time.
Wave-cut benches like these at Point Reyes National Seashore are examples of marine terraces.
Wilder Ranch State Park is another good location to see wave-cut benches and a succession of marine terraces.
 
Lake Shoreline Terraces
Lake shoreline terraces form from the rise and fall of water levels in inland lake basins. When lake levels are stationary, wind-driven wave action will carve steps into the side of the hillside. If the lake level rises or falls, and becomes stationary again, a new shoreline terrace will form. Shoreline terraces can form if a lake basin fills to capacity and spills into an adjacent basin, or if faulting, glaciers, landslides, or other geologic activity changes the base level or streams and groundwater entering and exiting a lake basin. Shoreline terraces are used to reconstruct the climate history of the southwest.
Shoreline Butte is at the southern end of Death Valley. The prevailing winds blowing southward through the valley produced waves in Lake Manly, the great lake that filled Death Valley during the Ice Ages.
Click on photos for a larger view.
Stream Terraces
Step-like terraces along valleys form as stream erosion switches back and forth between periods of down cutting versus valley widening floodplain development. Stream down cutting is typically a result in a change in a streams gradient (the slope down which it flows). A drop in sea level would cause an increase in gradient, and stream would respond by eroding downward into their channels. Other factor may influence stream gradients, including uplift or offset along a major fault, stream capture or diversion, and even changes in sediment supply or the volume of water flowing in a stream caused by changes in climate and vegetation cover.

Valley widening occurs when stream base-levels stabilized, and the streams gradient approaches horizontal. As a result streams will no longer carve downward, but rather will meander from side to side across the valley. Where the stream encounters the side of the valley it will erode into the base of the hillsides, and hillside material that falls or erodes into the stream will be carried away. In this manner, over time, streams will carve broad valleys. Base level can even rise, and stream channel and floodplain deposits will back fill the valley with sediments. Stream terraces and terrace deposits can be found throughout the western United States. Evidence suggests that their occurrence is related to the major climatic and sea level changes associated with the formation and melting of the great continental glaciers during the Ice Ages.
Stream terraces in Arroyo Seco Canyon, Monterey County.
Stream terraces in Afton Canyon, San Bernardino County.

Fault-Offset Terraces
Some features that look like terraces may actually be related to faults. Great earthquakes are often associated with surface rupture along fault zones. If there is a vertical component to the fault offset, an elevated terrace may result. Erosion gradually wears away at the disrupted surface offset. Studies of offset terraces help geologists determine the age, intensity, and frequency of past earthquake activity.
This offset terrace is along the Calaveras Fault in San Felipe Valley, Santa Clara County.
Old Mission San Juan Old Mission San Juan Bautista is built of an elevated terrace next to the San Andreas Fault. The dirt path that basically follows the trace of the fault scarp along the north side of the mission. This dirt path is the original El Camino Real. It is a walking path part of the San Juan Bautista State Historical Park. Desert palms grow near seeps on step-like faulted terraces along the Banning Fault in the Coachella Valley Preserve.
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8/28/2012