Geologic Evolution of the Kosciusko Alpine Area

Updated: Feb 1

This posting is a small section taken from the excellent “Kosciuszko Alpine Flora”, by Alec Costin, Max Gray, Colin Totterdell, & Dane Wimbush

“Unlike the mountains of New Zealand and New Guinea which are still being uplifted rapidly, those in eastern Australia are much older and have a complex evolutionary history. Some 450 million years ago Kosciuszko was covered by a large sea that extended over most of what is now eastern Australia. During this period (the Ordovician), extensive sediments were deposited. Remnants of these sediments — now much altered — are still to be seen as the slates, phyllites, quartzites and schists forming part of the Kosciuszko area between Rawson Pass and Watsons Crags. (Jeffe’s notes are in red.)

Periods of folding, uplift and sedimentation continued for millions of years during the Ordovician into the Silurian (440-410 mya (million years ago)) and early Devonian periods (409-363 mya) when intrusion of granites (intrusive rocks) and folding and uplift of the area above sea level occurred. The oldest of these granites — the most abundant rocks in the Kosciuszko (and Victorian Alps) Alpine Area — are about 390 million years old.

Many millions of years of relative stability of Earth's crust followed, during which the uplifted areas were weathered and slowly worn down to a fairly even paleoplain (or peneplain) surface with only a few of the most resistant parts, including some of the Kosciuszko peaks, remaining above the general level. This long period of crustal stability and erosion extended through most of the Carboniferous, Permian, Triassic, Jurassic and Cretaceous periods until the beginning of the Tertiary period about 65 million years ago (from about 360 mya to 65 mya. Dinosaurs were on earth from about 120 mya to about 65 mya).

The Tertiary period commenced an era of major uplift of eastern Australia during which the Kosciuszko area reached approximately its present elevation. The uplifting continued spasmodically until several million years ago, perhaps as recently as about one million years ago. It also caused extensive fracturing and faulting of the rocks and gave the rivers new erosive power. The major fracture patterns in the rocks provided zones of weakness along which many of the streams were able to cut down more rapidly to establish the stream pattern of long straight parallel courses which we see in the Kosciuszko area today: the upper Snowy, Crackenback, Guthega and Munyang Rivers provide good examples (as does the Bundara). From fossil evidence we know that the climate during much of the Tertiary period was warmer and wetter than it is now.

The Pleistocene period, commencing about two million years ago, was a time of generally colder climates throughout the world. At higher latitudes and altitudes glacial conditions developed, interspersed with warmer interglacial periods when the snow and ice cover largely or completely disappeared again. The Kosciuszko area was likewise glaciated, although apparently weakly. The cirques, moraines, erratics, lakes and polished pavements seen at Kosciuszko are products of these glacial conditions. Where the ice cover was thin or absent, the low temperatures also produced shattering of rocks, differential freezing and thawing of the soil with movement and accumulation of debris downslope, and other so-called periglacial effects both within and below the glaciated area itself (Seen more in the Victorian Alps). For the last few thousand years Kosciuszko has been virtually ice-free, although some of the late-lasting snowpatches sometimes persist for more than a year at a time. Because the Pleistocene period was such an important influence in most parts of the world on the evolution of the present landscape, soils, vegetation and animals, including humans, it has received considerable study.

Over the past few decades, studies of the shape of the edges of continents and continental shelves, and of the rocks which form them, indicate that many areas now widely separated at one time fitted together. Furthermore, studies of the magnetism of the rocks concerned show that the position of some of the land masses has changed, and absolute dating of the rocks based on their residual radioactivity allows definite ages to be put on the times of former connection and subsequent drifting apart. Such evidence indicates that the southern landmasses (including what is now Antarctica) were joined or were much closer together until about 45 million years ago. Australia — forming part of what we now call Gondwana — was then probably about 15 degrees further south than it is today. At this time, both Australia and Antarctica appear to have had a temperate climate, as shown by their fossil floras. In Antarctica, for example, the fossil remains include Southern Beech (Nothofagus) and conifers.

(There are many) similarities between individual species (of plants)— sometimes whole groups of related species are involved (across continents). Such distributions are generally interpreted as indicating former connection of the separated landmasses rather than chance distribution by long-distance dispersal as by birds, wind and water.

Erratic: A stone or boulder that has been carried by an ice­cap or glacier to a place where it rests on or near bedrock of a different composition.

Glossary:

Moraine: A landform composed of unsorted rock and soil debris deposited by an ice-cap or glacier.

Paleoplain: A fairly flat area produced by continued erosion.

Periglacial: Pertaining to frost action with alternate freezing and thawing under near-glacial conditions.

Slates, phyllites, & Schists: Rock types produced by increasing degrees of metamorphism, i.e. exposure to heat and pressure.”



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