What are Straths?

Straths are degradational valley floors. The actual meaning of this Scottish word is a valley with flat floor. Bucher reserved the term for such valleys whose level floor was due to lateral erosion by streams and other factors of degradation. The valley floor will mostly be in bedrock. If there is, uplift or increase in precipitation there will be invigorated down cutting along with lateral erosion causing the formation of strath terraces. As the diagram shows, a narrower and lower valley has developed within a broader valley. This is known as valley-within-valley or double-storey valley. The steep slope on the outer edge of the lower valley is called shoulder. The relatively level surface above the shoulder and being part of the original valley floor is called strath terrace. The term strath will be applied only if the surface above the shoulder is fairly wide, sometimes including several streams.

Strath is a peneplain or erosion surface in the incipient stage. Many areas show successive straths marking successive periods of still stand and uplifts. These are partial peneplains, because no still stand was sufficiently long to bring about the total planation of the area involved.

Thornbury tries to distinguish among the different features, which are narrow planation surfaces related to still stand of limited duration. The term ‘partial or level peneplain’ should apply to relatively extensive surfaces, which have been planated down near to the base level. The term berm and strath may be reserved for areally limited planation surfaces found in valleys.