Regolith of the Ridge and Valley Province
Residuum
Regolith formed inplace varies greatlywith bedrock lithology. Shales and some weakly cemented sandstones,such asthe Rocky Gap Sandstone in Giles County,VA, decompose while preserving structure, in a manner analogous to the formation of saprolite in crystalline rocks. Strongly cemented sandstones, however, such as the Tuscarora Sandstone, are less prone to decay in this manner. Instead, theytend to split apart along joints and bedding planes into boulders and cobbles, which generally are transported some distance downslope before they break down. Limestone produces a completely different kind of residuum, terra rossa. This red, clay-rich material, often in thicknesses of tens of meters, commonly overlies limestone in humid climates. Typically there is a very sharp contact between unweathered limestone and the overlying residuum, and the surface of the limestone may be irregular and pinnacled, indicating a spatially variable rate of dissolution. Generally terra rossa has been attributed to the insoluble residue left behind by dissolving limestone. More recent work,such as that of Olson and others (1981), however, makes a case that terra rossa contains not only residue from the underlying limestone, but also detritus from clastic rock formations that once overlayed the limestone. Terra rossa may be very old, having been let down hundreds of meters by solution. Plant-fossil assemblages as old as Cretaceous have been reported in these deposits. In some cases, there is evidence that terra rossa is primarily a transported sedimentary deposit, probably colluviated downslope into sinkholes (Hall, 1976).
Colluvium
The most common transported regolith associated with highlands in this province is what will be termed "boulder colluvium". Consisting of boulders and cobbles, usually composed of quartz sandstone set in a matrix of sand, silt, and clay, these deposits typically arefound on the flanks and along the bases of ridges capped by resistant clastic rocks, and cover large areas underlain by shales and carbonates. Thickness ranges from a fraction of a meter to more than 10 m. Where the colluvium overlies shale or carbonate, it frequency is separated from the bedrock by a layer of residuum. The bulk of this colluvium is relatively unweathered and brown in appearance (hue 10YR or 7.5YR [Munsell 1975]). Itcontains hard, angular sandstone stones. Ciolkosz and others (1986) noted that in Pennsylvania, soils developed in brown colluvium are similar in some respects to soils developed in late-Wisconsinan till, implying similar ages. Much less common in Pennsylvania is red (hue 5YR to 2.5YR), clearly older colluvium with decomposed sandstone clasts. Soils with such characteristics have commonly been assigned the age of the last interglacial (about 120 ka). In Pennsylvania, such red soil generally is buried beneath younger brown colluvium, which Hoover and Ciolkosz (1988) attributed to solifluction under a periglacial climate.
Near Mountain Lake in Giles County insouthwest Virginia, Mills (1987, 1988) used relative-age dating methods to demonstrate that boulder-colluvium age varies with topographic setting on the flanks of sandstone-capped ridges. The idea behind relative-age dating is to arrange deposits into relative order of age, even though no actual dates are available. Depositswith greater weathering and soil-profile development are presumed to be older than thosewith lesser degrees of weathering and soil-profile development. The specific properties used here to show differences in relative age include percent weathered stones, percent clay, and redness of the soil.
Note that although the ridges are linear features parallel to the local strike of the bedrock, on a finer scale they are corrugated by alternating hollows (small valleys, often streamless) and noses (spurs). Relative-age dating shows that the boulder colluvium in hollows commonly is much younger than that on noses. This age difference apparently reflects a longer residence time on noses than in hollows,which are much more geomorphologically active than noses. Runoff tends to converge in hollows, vs. diverging on noses, so that the debris becomes saturated much more often in hollows than on noses.The result isrelatively frequent debris slides and flows from hollows, sluicing out the colluvium and often scouring the hollow to bedrock. Erosion of noses is much slower and generally consists of slow movement of colluvium down sideslopes into the hollows. Transport is by windthrow and other creep processes. Vegetation in hollows is very different from that on adjacent noses (Stevenson and Mills 1999). Noses and hollows on mountain flanks are present in other physiographic provinces, but they are specifically addressed in this section because they are most regular and striking on ridge flanks in the Ridge and Valley.
Another finding from relative-age study of Mills (1987, 1988) was that the colluvium on noses lower on the mountain flanks, where slopes are gentler, is older than colluvium on steeper noses farther up the mountain. This difference probably results from faster creep rates and a greater tendency for slope failure where gradients are steeper. Unlike the finding in Pennsylvania by Hoover and Ciolkosy (1988) that red colluvium is usually covered by brown colluvium, in southwestern Virginia red colluvium is widespread on the surface of the noses.
Because of the abundance of large sandstone fragments shed by the sandstone-capped ridges, boulders are extensive on mountain footslopes. Boulder colluvium is plentiful, but deposits of boulders without interstitial fines are also common in the form of talus, boulder fields, and boulder streams. Boulder streams, consisting of linear collections of boulders in hollows, are the most common (Mills, 1988). Note that the term "stream" here does not necessarily connote the presence of running water. Many boulder streams occur in streamless hollows. Deep excavations show that, at a depth below a layer of open boulders equal to several times the diameter of an average boulder, spaces between boulders are filled with fines. Deposits of large boulders may affect the relative abundance of tree types (Mills and Stephenson, 1999). Some of these deposits may have been formed during the Pleistocene ice ages.
Encyclopedia ID: p1534
