Tuatara: Volume 11, Issue 1, March 1963
The Rate of Down-Wasting of Land Surfaces
The Rate of Down-Wasting of Land Surfaces
Calculations of the rate at which the surface of the lands is worn away by erosion have yielded some astonishing results. The older-fashioned method of calculation is to measure the amount of eroded material discharged by a river in the course of a year and to suppose it spread back as a layer of estimated thickness over the drainage basin. This method of measuring annual wastage allows the present-day rate of lowering of the land surface to be calculated; but the rate has not necessarily been the same in the past. Tricart (1961) has thus questioned the usefulness of values recently worked out by Fournier. Emphasising the fact that man's interference with the natural landscape by tillage has enormously accelerated erosion, he has strongly criticised Fournier's acceptance of an average lowering of the land surface by 1 metre in 2,500 years as ‘normal’. This rate — a quarter of a mile in a million years — he declares is ‘trop forte, beaucoup trop forte’, in fact ‘inadmissible’.
Expressed as 60 miles in 240 million years and applied, for example, to the eroded surface of ancient rocks that forms the landscape of the Southern Tableland of New South Wales, such lowering seems certainly inadmissible; but this does not mean that a more moderate estimate of even up to several miles in the same time based on or averaged from more conservative figures arrived at by other investigators is equally inadmissible.
Menard (1961) has supplemented the older method of measuring the rate of erosion by one that relies on calculation of the aggregate volume of waste derived from a region which has been deposited over a period of millions of years as accumulations in inland basins, as marginal-shelf and continental-slope sediments, and, in addition, the debris swept far away from land by turbidity currents and spread over oceanic abyssal plains. Both methods have been used by Menard with application to the denudation of specific sample regions among which are eastern North America (the Appalachian region), the Himalayan region drained by the Indus, Ganges, and page 27 Brahmaputra rivers, and the basin of the Mississippi. While for various reasons the past and present rates differ widely for some regions, for the Mississippi basin they are found to be almost identical. ‘For the Mississippi region the rate of erosion has not varied significantly for 150 million years, and this may be characteristic of regions of low plains’ (Menard, 1961, p. 159).
Though Menard hesitates to derive ‘world-wide average rates from these data with the small samples available’, it seems reasonable to regard the rate of erosion for the Mississippi region as very generally applicable, giving at least a conservative figure correct enough for many purposes. This rate Menard finds to be, for the last 150 million years, a wearing down of the land surface by 4.6 cm. every thousand years, a very impressive figure when one realises that it is derived from a lowering of the surface amounting to 6.9 km. (say 4 miles). The new method is of great value in that it affords an assurance that former estimates of past lowering extrapolated from annual river-load figures have not been altogether illusory.
One lesson that is to be learned from acceptable estimates of past lowering of the surface in specific regions is that these ancient terrains, and by implication other ancient terrains, have risen by an amount measurable in miles in the elapsed time. They must have done so to suffer that amount of erosion — whether gradually or by fits and starts is immaterial. In either case the imagination baulks at the task of unravelling the many changes of landscape form such a region has experienced as it has risen and suffered erosion. One could only picture the changes like the showing of a slow-motion cinema film.
Eastern Australia cannot be immune from such changes. It is regarded as a region in which remnants of very ancient landscapes survive, but it is not one that has retained its ancient form unchanged. The landscape is not ‘old’ in the geomorphic sense, and this testifies to upheaval. Yet there has been a tendency among geologists to ignore the implications of the enormous changes of form that must have accompanied vast upheaval and a consequent erosional loss of substance through many millions of years. To take an example, superficial and obviously young (? mid-Pleistocene) gravel deposits not far above present river levels in valleys near Canberra have been assigned an origin in the Permian period, implying that the landscape, though it is mature with quite strong relief, has been immune from erosion for some 250 million years, remaining practically unchanged in form for all that time.
Menard, H. W., 1961. Some rates of regional erosion. J. Geol., 69, pp. 154-161.
Tricart, J., 1961. Review of F. Fournier, Climat et érosion, Paris, 1960. Rev. Géom. dyn., 12. pp. 53-54.