Horst

HORST IS THE German word for a high-nesting area for a bird, so the term infers a high elevation. Early geomorphologists fittingly applied the German word to a fault block mountain range. The faulting and vertical displacement is due to tension stresses in the crust. Tension causes crustal rock to spread apart and break to form steeply inclined normal faults.

The fault lines are normal to the direction of spreading and parallel to each other at the surface, but their planes dip away from each other beneath the surface. A horst is left standing either by sinking of the crust on either side of a pair of normal faults or by physical lifting of a crustal block between the faults. Geomorphologists often use the German word graben (which means “trench” or “grave”) for the low-lying block between two horsts.

Horsts have either symmetrical or asymmetrical profiles. Equal rates of vertical movement along the parallel faults produce a symmetrical profile. Horsts of this sort may have flatlike tops, but they are more likely in various stages of erosion. Horsts have asymmetric (tilted) profiles when the vertical movement is along only one of the bounding faults. The resulting mountain has a steep escarpment on one side and a gentle back slope on the opposite side. The Sierra Nevada of CALIFORNIA are a classic example of a asymmetrical fault block mountains; their steep eastern escarpment faces NEVADA and the gentler western back slope faces California’s Central Valley.

Horsts (ranges) in combination with grabbens (basins) form a basin-and-range topography; Nevada is in the middle of this terrain type, which extends into IDAHO, OREGON, California, UTAH, ARIZONA, and NEW MEXICO as well as northern MEXICO. Countless horsts interrupt the landscape of the region. For example, California’s Owens Valley (a graben) separates the Sierra Nevada from the White Mountains (also a horst). Another example is the Panamint Mountains and Armagosa Mountains; they align on either side of Death Valley. Countless horsts occur in other parts of the world, wherever normal faulting has occurred. The Vosges Mountains of France, the Harz Mountains of GERMANY, the island country of TAIWAN, and several highland ranges of KENYA and ETHIOPIA in eastern Africa are horsts. The SINAI PENINSULA between waterfilled grabben of the Gulfs of Suez and Aqaba is another prominent example.