Véase T H Marshall, 195, en el que se reproduce el texto original de 1949.

An active pediment is defined in this study as follows: 1. A 'pediment is a terrestrial erosional feature. This is the only genetic aspect of the definition. It is meaningful and convenient to distinguish between erosional and depositional features in spite of the fact that they are interdependent aspects of landscape evolution. On the one hand we are concerned with the breakdown of rock into sizes which can be moved, involving the study of lithological, structural and

time controls on weathering, and the mechanisms responsible for

initiating and maintaining the movement of such material. On the other • hand we are concerned with the loss of efficiency of the transporting process, and the manner in which it releases material and the forms of deposition. The distinction made between erosional and depositional landforms is thus primarily a question of the nature and timing of the acquisition of its form: an erosional landform owes its form to

processes which postdate those which formed the rock; a depositional landform is characterised by the contemporaneous development of rock and form.

However, a depositional landform can be transformed into an erosional landform (e.g. Dresch 1957), but a rigorous definition of when this occurs is hazardous since the discontinuous nature of erosion in both time and space makes it essential to define the time and space dimension in question. For our purposes, a landform is a pediment if

(at a fixed point in time) it satisfies the morphological requirements listed in 2 - 6 below inclusive, and, taken as a whole, it has acquired these characteristics by processes postdating those forming the rock in which it is developed.

As a corollary we must note that a pediment is never purely tectonic relief. However, this definition does not exclude landforms where erosion has taken advantage of the internal structure of the

terrain (Cotton 1968) to form a pediment (e.g. parallel retreat controlled by a resistant caprock).

The remaining parts of this definition concern pediment morphology and not genesis.

2. A pediment is a footslope to higher and steeper ground. 3. A ,Tbedrock pediment surface" may be covered totally or in part by allochthonous material to form a "subaerial pediment surface", or it may be devoid of any cover whatsoever. No precise limit should be set to the depth of burial acceptable in identifying a bedrock pediment, except that pedimentation may no longer be active when the buried surface ceases to control the overall form of the visible land surface. However, renewed erosion may re-expose a fossil pediment.

4. A pediment has a characteristic profile normal to the hillfront: the slope is at a low angle relative to the steeper ground of which it is a footslope. It is generally inclined at only a few degrees, although in exceptional cases (e.g. Fair 1948) it may be appropriate to use the

term for slopes up to 13‘. The profile normal to the hillfront is smooth

and truncates structural and lithological variations, but a faint concavity or convexity or both may be present in the same locality (e.g. Twidale

1956). Characteristically the nickline between the footslope and the

hillslope is angular. An attempt is made in this study to quantify this angularity, but the problem remains as to where the distinction between

angular and non-angular breaks of slope should be made. Such a

distinction awaits comparison with breaks of slope on other (non-pedimented) landforms.

5. A pediment has a characteristic profile parallel to the

hillfront: it is smooth and lacks significant irregularities due,

for example, to stream incision.

6. A pediment therefore has a characteristic shape when considered

in three dimensions: it is a generally planar low angle footslope

surface with or without a faint concavity or convexity which truncates structural and lithological variations.

To summarise, a pediment may be defined as follows:

"A pediment is a terrestrial erosional footslope surface inclined at a low angle and lacking significant relief in all three

dimensions. It usually meets the hillslope at an angular nickline3 and may be covered by allochthonous material. "

It is important to note that a pediment may be destroyed by the initiation of linear erosion. Relative relief will increase, and the processes destroying the planar shape may be more important than those creating or preserving it.

The merits of the definition given above are twofold. First, with the justifiable exclusion of purely depositional, non-terrestrial, and tectonic landforms, the definition is non-genetic. Otherwise the landform is not restricted to any process, lithology, climatic

environment or size. Second, the morphological attributes distinguish pediments from other landforms.

There are two limitations to this definition. First, Nature rarely conforms to strict boundaries: while endpoints may be

distinguished readily (e.g. a perfectly planar pediment or a thoroughly irregular footslope) the continuum between these extremes may be so gradual that it is far from clear when one has become the other. Therefore, while it may be very difficult to define the degree of angularity of a pediment nick or the tolerable imperfections on a

surface before it ceases to be "planar", it may in fact be pointless to do so. A pediment is therefore a concept or an ideal; it would be better to describe the degree of pedimentation rather than to decide whether or not a landform is a pediment.

The second limitation is that there is a disadvantage in introducing the genetic restriction "terrestrial" into the definition because some pediments have been claimed to be wave-cut platforms

(e.g. the pediment beneath the Morown Cliff in West Kimberley

(J.N. Jennings, pers. comm.)), and it would be better to leave genesis to investigation. However, this could be seen as too great a departure from conventional usage to be acceptable, and this restriction is