FAULTED STRUCTURES 



815 



Tenns applied to rock masses formed by or bounded by faults — but 

 not topographically distinguishable from surrounding masses. 



1. A horst is a mass geologically elevated relatively to the sur- 

 rounding region and separated from it by faults. 



2. A fault basin is a region geologically depressed relatively to 

 the surrounding region from which it is separated by faults. 



3. A fault block is a mass bounded on at least two opposite 

 sides by faults. It may be geologically elevated or depressed rela- 

 tively to the adjoining region, or it may be geologically elevated 



relatively to the region on one side and depressed relatively to that 

 on the other. 



4. A fault ridge is a relatively elongated fault block lying be- 

 tween two faults with roughly parallel trends. 



5. A fault trough is a relatively depressed (geologically) fault 

 block lying between two faults with roughly parallel trends. 



Terms applied to the topographic expression of faults. 



1. Fault scarp — a scarp or cliff presenting the original surface 

 form of the displacement. 



2. Graben. or fault scarp valley — a long, narrow topographic 

 depression, the surface expression of a new fault trough. Ex. 

 Rhine Graben ; Purgatory Chasm, near Sutton, Mass. It is bounded 

 on both sides by fault scarps facing inward. 



