Pediments

Australia: The Land Where Time Began

Pediments

Pediments are bedrock surfaces that are gently sloping, 0.5-7^o, though typically 3-4^o, and are smooth and gently dissected, with few stream lines. In longitudinal section many are slightly concave, though they are often rectilinear and occasionally convex. The transition from upland and plain tends to be more abrupt in arid and particularly semiarid areas than in more humid areas. The piedmont angle is the critical sharp transition between pediment and backing scarp.

There are 3 types of pediment, covered pediment, rock pediment and mantled pediment. Covered pediments occur in sedimentary terrains, where they are covered by a thin cover of transported coarse detritus. Mantled pediments occur in granite country and are covered by a mantle of weathered material. In places this weathered material has been stripped away leaving a rock platform, a rock pediment.

Pediments are present in many parts of the world, some of the cold places being the Pyrenees, Utah and Alaska, as well as monsoonal places such as Japan and Korea, though according to the authors (Source 1), they are believed by most to be best developed in arid and semiarid areas, where they are considered to be typical. There are, however, some who categorise all plains that have been moulded by water to be of this type.

According to the authors (Source 1), geomorphologists have been debating mainly 3 aspects of pediments for many years. These are the process that produced such smooth bedrock plains, whether pediments extend, eventually coming to occupy a large portion of the landscape and the origin of the piedmont angle.

It has been suggested by some that pediments are surfaces remaining after the slopes have retreated, there being a strong suggestion that some (structural) conditions, such as scarp retreat, have occurred and in still continuing. The evidence from the field in fold mountain belts and in regions of granite, points to the lowering of plains, etching out of piedmont zones weathering and erosion, the scarp retreat in such areas being minimal. On the Eyre Peninsula granite inselberg flanks have been worn back by only a few metres over various phases or cycles of erosion over millions of years. The authors (Source 1) suggest that by attributing it to scarp retreat or scarp retreat and pedimentation the question is dodged, still asking "what process have been at work?"

Laminar flow, the action of thin water sheets, has been suggested by King to be responsible for the shaping and smoothing of pediments. Twidale & Campbell suggest it is doubtful that laminar flow exists to any great extent in nature, because of the minor irregularities, such as stones and plants, would induce turbulent flow, and they suggest it is difficult to explain how such thin sheets of water could transport the cobbles and boulders that are present on some pediments.

It has been suggested that some pediments have formed as a result of sheet floods (W.J.McGee). Where rivers in flood emerging from uplands break their banks, spreading over the lowlands, develop a distributory habit, leading to an increase of wetted perimeter, decreased hydraulic efficiency and the formation of shoals by the deposition of bedload. The channel is said to divaricate, divided and subdivided again and again, shaping of the spectacular pediments fringing part of the Flinders Ranges result from such spreading streams.

Low-angle bedrock fans are formed, apparently largely by lateral corrasion, when streams in flood erode soft sediments in the lowlands. A mantle of cobbles and boulders are deposited at the same time, the stream deposits covering the irregularities in the cut bedrock surface that result from erosion, hence the typically smooth pediment surface and the fan shape of the individual forms.

This explanation is consistent with the evidence from the field in many areas, according to Twidale & Campbell, it is not entirely consistent with the field evidence from a number of other places. Pediments form fringes around very small residual nubbins or inselbergs in some places, such as Naraku in northwestern Queensland, the western Pilbara, northwestern Eyre Peninsula and the Pinnacles, Broken Hill, western New South Wales. In these places Twidale & Campbell consider a better explanation for the landforms in question to be weathering, occasional wash, rill work and gullying, operating over very long periods of time. They consider rock pediments to be etch equivalents of mantled forms, the regolith having been stripped away to expose the surface of the bedrock, with basins and preferentially weathered fracture zones.

In places such as the Flinders Ranges, mountain fronts are fringed by pediments, and around isolated hills or inselbergs they form low-angle cones. Twidale & Campbell doubt the suggestion that they extend to occupy the entire landscape. There are extensive pediments in some of the basins in the American southwest, though in granite terrains on the Eyre Peninsula, and sedimentary regimes, such as those in the western Cape Fold Belt, near Cape Town, South Africa, pediments that developed in the piedmont zone give way to rolling or undulating plains downslope. They suggest this may be the result of the protective influence of permeable depositional cover in the piedmont zone, with raised flow volume downslope that causes increased density in the stream network, as well as closer dissection of the unprotected bedrock.

The third controversial point. There have been many explanations for the piedmont angle. One such explanation, suggested by workers in the American Southwest, where faulting is common, is that the piedmont angle results from tectonic dislocation, i.e., the angular junction between adjacent fault blocks, the scarp being worn back leaving a low relief surface at its base in the piedmont. They suggest that evidence in disagreement with this hypothesis as a general explanation of the piedmont angle is the presence of this feature in areas with no faulting.

In another suggestion, lateral corrasion has been proposed as a significant factor involved in piedmont angle development, rivers emerging from uplands to arid plains, meandering along the mountain front, undercutting and steepening the mountain front, resulting in the formation of an angular front. Evidence of this suggestion, in the form of distinct scallops, the scars left by meander loops, are not present along the upland margin. There is also the observation that when rivers debouch on to the plains, adopting a distributory pattern, they continue flowing towards the centre of the local drainage basin away from the uplands. The piedmont angle occurs in the absence of major streams, as well as around isolated uplands that are too small to generate streams of concentrated drainage.

The mountain front is indented and valleys are being widened as intervening ridges are being reduced by lateral corrasion in areas such as the Wasatch Mountains of Utah, where there are many closely spaced rivers flowing from the uplands. In parts of Australia, corrasion is locally important, in places such as the Fitzroy Basin, Western Australia, as well as near many major river outlets. Lateral corrasion by closely spaced distributory streams is an important factor involved in the steepening of the sides of valleys and the indented upland front, as occurs in the Krichauff Ranges in the Northern Territory, as well as other uplands that are nearby, that are partly or wholly underlain by impermeable rocks.

King, among others, suggest that the piedmont angle is the result of contrasted processes. According to this view, quarrying of the scarp face is caused by turbulent flow, the flow becoming laminar (sheet) when the stream debouches on to the smooth pediment, where the change of process occurs, the angular break of slope develops. A problem in this context is laminar flow, and if the different processes occur they can be interpreted as resulting from pre-existing topographical features, not as the reason for that topography, cause and effect possibly being confused.

The authors suggest there is no doubt that structure is an important factor in the formation of the piedmont angle, often located at lithological or structural junctions. The piedmont angle is present at the base sandstone or limestone escarpments in the Flinders Ranges. It occurs at the bases of inselbergs in many granite areas which have a fracture density low in comparison to that of the surrounding plains underlain by granite blocks that are closely and moderately jointed. Slope development is also influenced by the presence of a resistant capping, which also assists in the maintenance of steep slopes that are partially responsible for the abrupt break between hill and plain.

There is widespread evidence of localised concentration of weathering, and therefore erosion, of lower slopes. Weathering that is especially intense affects the lower slopes, particularly the scarp foot zone, on the Eyre Peninsula, as well as throughout the Flinders Ranges. Kaolin and similar minerals have resulted from complete weathering of the local bedrock in the scarp foot in many places, though there is virtually no alteration a relatively short distance from the scarp, that are presumably similar strata. As a result of scarp-foot weathering, a narrow moat or gutter forms around the uplands that is especially well demonstrated around the bases of granite inselbergs on the northwestern Eyre Peninsula. scarp-foot moisture also results in steepened or flared slopes.

Sources & Further reading