Ancient Australian Landscapes

Australia: The Land Where Time Began

Ancient Australian Landscapes

Australia is the continent of ancient landscapes (Victor R. Baker, The University of Arizona ). Europe, Asia and North America are dominated by younger landscapes. In South America there are the Andes Mountains, that are relatively young, there are rift zones and the Atlas Mountains in Africa, and Antarctica is mostly covered by glacial ice that is currently active. In contrast to the other continents all of Australia is characterised regionally by landscapes dating to at least the Tertiary, and as argued by the author¹, to the Mesozoic. There are younger landforms that have been superimposed, such as coral reefs, shore platforms and alluvial fans. Indicators of their great geological age are preserved in plains, that are regionally extensive in Australia, as well as erosion surfaces.

According to the author¹ one of the most intriguing problems posed by the Australian landscape relates to the great age of many of its component parts. Areas such as Kangaroo Island, the Flinders Ranges, the Gawler Ranges, Yilgarn Craton or Block, Kakadu, and many areas in the Eastern Uplands are apparently so old that they are impossible in terms of traditional theory. The ancient landscapes of Australia are an aspect of the continent that is very unusual. Some elements of the Australian landscape are among the oldest in the world. The implications of such possible antiquity are at odds with conventional wisdom. It is of interest to stratigraphers as they are interested in erosional aspects as well as depositional aspects of chronologies of the past, and also to palaeobotanists, as remnants of older landscapes may prove to have been plant refuges (e.g., Hopper et al., 1996; Fay et al., 2001).

A widely accepted view is that the landscapes of the Earth are relatively young, the conclusion being well-founded in everyday observation of natural events, and in common sense. Rocks exposed at the surface of the land are continually being eroded by wind, rivers and waves, as evidenced by rivers that are turbid, coloured with mud and clay, following heavy rain. Cobbles and boulders that have been carried by rivers in flood and high flow rates become visible when the water levels of the rivers fall, the sediment having been picked up in the upstream sections of rivers, an indication that the land surface along the upstream reaches of the rivers are being eroded. River flow rates are variable, with some being episodic and unpredictable, others flowing intermittently or variably, though regularly so. There are periods of flood and low stand even in rivers that flow permanently. Rivers and streams can be regarded as flowing constantly on geological time scales; hence the conclusion that the surface of the Earth is being constantly changed.

More than 200 years ago James Hutton (1788, 1795), a Scottish farmer and scientist, thought of by some to be the 'father' of modern geology, considered that there have been long periods of gradual change, during which the surface of the land was modified by erosion in some places and deposition of sediment in others, though these periods have been interrupted by catastrophic events (see also Baker, 1973, Hunt, 1990; Ager, 1993). Hills have been lowered and sediments have accumulated to form new strata. According to the Hutton's concept no part of the surface of the Earth can have survived long modification by the external forces such as water, ice and wind. Based on this concept geologists and geomorphologists have believed that all landscapes are youthful, in geological terms (e.g. Wooldridge, 1951; Linton, 1957). Some have suggested that no landscapes were formed more than about 2 Ma (e.g. Ashley, 1931; Thornbury, 1954,p. 26), while others have suggested that some remnants of landscapes could possibly have persisted for up to 60 My (e.g. Brown, 1980).

And this is true for most parts of the continental surfaces including the great riverine and desert plains occupying vast areas of central Australia, with some being relatively young at only a few thousands, or few hundred thousand years old. In keeping with the models of landscape evolution that have been introduced over about the past 100 years acknowledged the existence of constant change, implying that the resultant land surfaces are youthful. The Davisian model (1899, 1909) was based on the lowering of slopes, downwearing. Building on the work of earlier scientists such as (Fisher, 1966; Jutson, 1914; Holmes, 1918), King (1942) considered that the slopes of valley sides were worn back within close limits parallel to themselves, but achieved baselevelling, or the reduction to lower relief of the landmass, over a few 10s of millions of years.

It was suggested (Hack & Goodlett, 1960; Hack, 1960) that a steady state of slope development adjusted to valley incision could be attained by a surface, and undergoing constant erosion without the shape changing significantly. Other schemes of development have been devised in which various combinations of uplift, stream incision and valley-side development were considered (e.g. Kennedy, 1962). Pronounced river incision combined with minimal wasting of divides, is involved in one of Kennedy's models, but no spatial or temporal scales are indicated.

The interplay between tectonics and exogenetic agencies were the primary interest of Walther Penck, and he took this into account in his interpretation of slopes and planation surfaces (Penck, 1924, 1953). He suggested, therefore, that increasing uplift was indicated by convex valley-side slopes, neglecting the controls exerted by local and regional baselevels. He suggested that slopes be regarded from base upwards (see e.g. Bremer, 1983). He recognised that were 2 ways in which plains could evolve, either by lowering by streams (Endrümpf), or by the interplay between crust that is rising and rivers that are incising (Primarrümpf), neither of which imply long-term survival.

Baselevel control is assumed in most of theses landscape development models. Areas on divides are older than areas near the river mouths of incised rivers, with younger surfaces being initiated and from which they gradually extend. As this takes time, in respect of an extensive continent, the older landscape elements persist on divides, particularly if scarp retreat has been prevalent, as remnants of the older, incised surface are thus allowed to persist until late in the event sequence. Such a sequence or 'cycle' (as the end result is similar to the initial surface) lasts 30-40 My (e.g. Schumm, 1963). The very old elements persisting in some contemporary landscapes, this is still not enough time to accommodate them.

Some geologists, most of whom were working in the Southern Hemisphere, reached the conclusion that some elements of the landscape are much older than is allowed according to conventional theories and models, indicating that the youthful view of landscape was not applicable universally. It was suggested by a British geologist (O.T. Jones, 1931) for example, that the surface of the uplands of central Wales were from the Triassic, from about 200-250 Ma. The evidence of great age from Wales and southern Africa was inconclusive, though it was suggestive (e.g. Willis, 1936; du Toit,1937, pp. 229-230; Wellington, 1937; Dixey, 1938, 1942; King, 1942, 1962). A Cretaceous age of the Schooley Surface of the Appalachians has been entertained for many years, though conclusive evidence has not been found (e.g. Bascom, 1921; Bliss & Knopf, 1924); but see Poag & Sevon, 1989). Beginning in Australia in the 1920s and early 1930s evidence was noted that pointed very conclusively to landforms and landscapes that were from more then 60 Ma (Hossfeld, 1926; Craft, 1932, 1933; Hills, 1934, 1938).

Many of these old surfaces were of the etch type, as they had been stripped of any regolith that had developed during their planation, a crucial point, as a bare rock is more likely to persist than it would if it was covered by soil or regolith that retains moisture. Though of some antiquity, the etch concept (Twidale, 2002), was not appreciated widely until the middle of the 20th century or later, the geological world at large was not impressed by such evidence and argument, taking little notice of these suggestions that appeared to be absurd. In has been confirmed in southern Africa (e.g. Partridge & Maud, 1987), and Australia (e.g. Twidale, 1994) by geological mapping and advances in rock dating technology, and in some instances being demonstrated afresh that landscapes with an age of several scores, or even a few hundred million years old have survived to the present.

Questions have been raised by such very old elements of landscapes as to why and how they have persisted. There is also the implication of a decrease in the amplitude of relief that involves either downwearing or backwearing within the period of a cycle or sequence of change in conventional models of the development of landscapes. However, it is suggested by field evidence, that the amplitude of relief - the vertical height between the highest and lowest point in the landscape - has increased over long time periods as the plains and valleys have been eroded faster than the uplands.

An analysis of Australian landscapes demonstrates that there are 3 major conceptual problems.

Some landscape elements are older than is feasible in terms of the conventional models of the evolution of landscapes.
What characteristic of very old landscapes allowed them to survive?
Some landscapes have developed an increase of relief amplitude over time. What is the evidence?

Answers to these 3 problems have a bearing on the general geomorphological theory and the way landscapes are viewed. They implicitly pose a 4th question, how typical the surface of Australia is of the global picture?

Sources & Further reading