Fluvial terrace
Fluvial terraces are elongated
Types
There are two basic types of fluvial terraces, fill terraces and strath terraces. Fill terraces sometimes are further subdivided into nested fill terraces and cut terraces. Both fill and strath terraces are, at times, described as being either paired or unpaired terraces based upon the relative elevations of the surface of these terraces.[4]
Fill terraces: Fill terraces are the result of an existing valley being filled with
Cut terraces: Cut terraces, also called "cut-in-fill" terraces, are similar to the fill terraces mentioned above, but they are erosional in origin. Once the alluvium deposited in the valley has begun to erode and fill terraces form along the valley walls, cut terraces may also form below the fill terraces. As either a stream or river continues to incise into the material, multiple levels of terraces may form. The uppermost being the fill terraces and the remaining lower terraces are cut terraces.[5]
Nested fill terraces: Nested fill terraces are the result of the valley filling with alluvium, the alluvium being incised, and the valley filling again with material but to a lower level than before. The terrace that results for the second filling is a nested terrace because it has been “nested” into the original alluvium and created a terrace. These terraces are
Strath terraces: Strath terraces are the result of either a stream or river downcutting through bedrock. As the flow continues to downcut, a period of valley widening may occur and expand the valley width. This may occur due to an equilibrium reached in the fluvial system resulting from: slowed or paused uplift, climate change, or a change in the bedrock type. Once downcutting continues the flattened valley bottom composed of bedrock (overlain with a possible thin layer of alluvium) is left above either a stream or river channel. These bedrock terraces are the strath terraces and are erosional in nature.[6]
Paired and unpaired terraces: Terraces of the same elevation on opposite sides of either a stream or river are called paired terraces. They occur when it downcuts evenly on both sides and terraces on one side of the river correspond in height with those on the other side. Paired terraces are caused by
Applications
Fluvial terraces can be used to measure the rate at which either a stream or river is downcutting its valley. Using various dating methods, an age can be determined for the deposition of the terrace. Using the resulting date and the elevation above its current level, an approximate average rate of downcutting can be determined.[6]
See also
- Bench (geology) – Long, relatively narrow land bounded by distinctly steeper slopes above and below
- Landform – Feature of the solid surface of a planetary body
- Musashino Terrace– Large tableland in Kantō region, Japan
References
- ^ Fairbridge, R. W., 1968, Encyclopedia of Geomorphology. Reinhold Book Company, New York.
- ^ a b Blum, M., and T.E. Tonqvist, 2000, Fluvial responses to climate and sea-level change, a review and look forward. Sedimentology. v. 47 suppl. 1, pp. 2-48.
- ^ ISBN 0-13-669762-3
- ^ Pazzaglia, Frank J., in press, 9.2.3 Fluvial Terraces, Archived 2010-08-01 at the Wayback Machine in Wohl, E., ed., Treatise of Geomorphology. New York, NY: Elsevier.
- ^ ISBN 0-13-860958-6
- ^ ISBN 0-632-04386-5