Streams of the Ridge and Valley Province
Several streams rise near the crest of the Blue Ridge and flow northwesterly across this province. Generally, however, when they reach the Ridge and Valley Province they turn southwesterly and flow along strike valleys parallel to the general Appalachian trend, as illustrated by the French Broad River is an example.
The sole exception is the New River, which not only cuts across the Blue Ridge but continues northwestward across the Ridge and Valley Province (and across the Appalachian Plateau as well). Because this flow seems to follow the path of inferred flow during the Paleozoic, some researchers have maintained that the New Rivers course has retained the same direction of flow since the Paleozoic. The river thus may predate the topography. That is, when the stream first formed, the topography was such that a northwesterly course was the preferred direction. Subsequently, the topography has changed and become unfavorable for this direction of flow, but the New River has been able to maintain its original course. If this interpretation of the New Rivers history is correct, it implies that the present-day course of the New River may be as old as 500 million years. Of coursewe refer hereonly to the general course of the river, not the rivers exact position. Rivers change greatly through time - they shift sideways, abandon meander loops, and change their geometries. As several kilometers of rock have been eroded away since the original course of the New River was established, undoubtedly the exact location and appearance of the river have changed.
One sometimes sees references to the New River as the "oldest river in North America", and the "second oldest river in the world." Given the problems of interpretation already mentioned, such statements are even more difficult to evaluate. Probably the best that can be said with any confidence is that the New River is one of the oldest rivers in North America. Apparently, general statements by geologists on the antiquity of the river somehow became transformed into the more specific "oldest" and "second oldest" statements (Morgan and Mayfield 1994).
Although, structure exerts some control on drainage in the Piedmont and Blue Ridge, control is much stronger in the Ridge and Valley, owing to the relatively simple structures here, as well as the much greater differences in the erodibility of bedrock. On several scales, the trellis drainage pattern (in which a master stream has numerous tributaries joining at right angles) is very common in the Ridge and Valley. Where a stream following a strike valley is considered the master stream, the numerous streams draining the flanks of adjacent strike ridges constitute the tributaries. On a larger scale, where a large stream cuts across the strike ridges, streams flowing in the strike valleys constitute the tributaries and join the master stream at right angles. The New River is an example.
Stream terraces form when streams cut down, abandoning their floodplain and establishing a new one at a lower level. The remnants of the former floodplain are called terraces. The stream may go through the downcutting processes several times, producing terraces at multiple levels. Terraces may be paired or unpaired. Paired terraces are those with corresponding elevations on both sides of the streams, allowing ready correlation. For example, terraces 5 m, 10 m and 20 m above modern river level (AMRL) might occur on both sides of a stream. Unpaired terraces are thosethat differ in elevation on the two sides. For example, one side might have terraces at 5 m, 10 m, and 20 m, whereas the other sidemight have them at 10m, 15 m, and 25 m. Such terraces can be formed by rivers as they swing back and forth across a floodplain, because streams commonly cut down at the same time that they are moving laterally. For example, a stream may start out on one side of the valley at a certain elevation, and by the time it has migrated to the other side its floodplain may be 5 m lower than on the side where it started. Thus, were this floodplain to be abandoned to produce terraces, the terrace on one side of the stream would be 5 m higher than the one on the other side. Unpaired terraces are likely to be less continuous than paired ones. Generally, paired streams are more indicative of systematic changes in the stream, such as might be cause by climatic change or tectonic uplift. Unpaired streams can be formed without such influences.
Hack (1965) attempted to correlate terraces along the Middle River Valley of the Shenandoah Valley inVirginia, and found that only terraces within 9 m AMRL were sufficiently continuous to make for any great distance. Delcourt (1980) and Delcourt and others (1986), however, succeeded in correlating much higher (up to an elevation AMRL of 30 m) terraces along 54 km of the Little Tennessee River in Tennessee by constructing longitudinal profiles along the floodplain and terraces.
Terraces at some locations in the Ridge and Valley reach elevations of more than 200 m AMRL (King 1950; Bartholomew and Mills 1991). The age of these is far beyond the range of radiocarbon dating, and dating and correlation of such terraces has been difficult. Study of the undated high terraces is made even more difficult by the carbonate bedrock that underlies many terraces in this province, obscuring the continuity of the alluvial surfaces as it undergoes karsification. Mills and Wagner (1985) attempted correlation of high terraces along the New River in southwestern Virginia. Realizing that individual terraces could not be reliably mapped, the authors attempted to identify preferred terrace elevations by means of an area-altitude analysis of alluvial deposits. Results showed that an altitudinal mode of terraces occurs at 0-12 m AMRL, a low occurs between 12-30 m, and a second mode at 30-49 m. Despite the lack of numerical chronologic control, this altitudinal analysis documents an episode of downcutting (incision of more than 18 m) separating two major episodes of floodplain formation (lateral cutting). Relative-age dating of terrace deposits based on percent clay, soil color, and heavy-mineral analysis showed that there is a dramatic increase in weathering for sites above about 30 m AMRL, providing additional evidence for a discontinuity in the rate of stream downcutting.
Only recently have numerical dates been obtained from these high terraces. Granger and others (1997), by dating fluvial quartz in caves along the walls of the New River Gorge using the cosmogenic isotope burial dating technique, found incision rates of 20-30 m per million years, thus indicating ages of several million years for the higher terraces.
Encyclopedia ID: p1533
