Contributed by Dr. John F. Lindsay former local geologist at AGSO
The city of Canberra is built largely on rocks of Ordovician age (420 million years old). At the time the rocks formed, and in fact for most of the Palaeozoic era, the eastern margin of the Australian continent was an active margin. That is, the Australian continental plate was overriding the oceanic crust to the east. This resulted in the formation of troughs seaward from the continent and in the development of a volcanic arc landward from the margin. The best modem analogy is perhaps the western Pacific, especially the area around Japan and China. Canberra during the Ordovician was then a busy place with active volcanoes to the west of the city while beneath the city was a trough which varied in depth as the volcanoes poured out occasional streams of lava and vast quantities of ash (sometimes the trough was filled with ocean water sometimes there were lakes). Towards the end of the Palaeozoic the region entered a long period of mountain building. The area was uplifted and large granite batholiths were intruded as a high temperature liquid into the core of the mountain range (the granites can be seen around Tidbinbilla). Thus, since the ancient rocks of the Canberra area were deposited there has been a long period of erosion to arrive at our present landscape. As a consequence our soils are old and most of the readily soluble minerals have long since been dissolved and carried away in the streams.
Cooleman Ridge (our local park system) sits almost entirely on Ordovician volcanic rocks referred to as the Laidlaw Volcanic Suite. This suite is divided into two units or formations which are slightly different in composition. The older unit is referred to as the Deakin Volcanics and the younger unit as the Laidlaw Volcanics. The lavas and volcanic ashes in these formations are referred to as rhyolites and dacites. Chemically, by comparison with other volcanic rocks, they contain a relatively high proportion of silica (SiO2) and are thus "acid" volcanics which are similar in composition to granites formed at much greater depths in the earth's crust.
At the northern end of the ridge, just outside the park in the pine forest on Narrabundah Hill, the Deakin Volcanics are exposed. They appear as rather poor outcrops of rubbly boulders scattered about the hill but are perhaps best seen at the trig station on top of the hill. The rocks consist of ignimbrites, tuffs and water-lain sediments. Ignimbrites are formed when gas-supported flows of extremely hot volcanic ash pour down the sides of active volcanoes as rapidly moving clouds. As the ignimbrites came to rest they were still so hot that the ash fragments welded together forming a solid rock that is often difficult to distinguish from lava. Volcanic ash by contrast was ejected well up into the atmosphere where it cooled before settling out to form ash layers over large areas of the countryside. Ash layers are often eroded by subsequent rains and carried into local lakes to form water-lain sediments. Anyone wanting to see this unit in more detail, and prepared to risk life and limb, should walk the road cutting along the Tuggeranong Parkway from the Cotter Road overpass to the Hindmarsh Drive overpass. This unit underlies much of the Weston Creek area.
Walking south from the Narrabundah Trig Station you drop down into the saddle forming the headwaters of Bulgar Creek (where Hindmarsh Drive becomes Eucumbene Drive) before climbing back up the hill towards Cooleman Trig (above Chapman) and into the park. This saddle forms the boundary between the Deakin Volcanics and the Laidlaw Volcanics. The Laidlaw Volcanics form the foundations for part of Rivett, all of Chapman and parts of Stirling and Fisher. However, most importantly they underlie most of Cooleman Ridge to Mount Arawang and beyond to the south. Like the Deakin Volcanics the Laidlaw Volcanics consist in large part of ignimbrites but also include small amounts of volcanic ash as well as sediments deposited in the ocean rather than in lakes. In general, to the casual observer the two units are quite similar. The Laidlaw Volcanics generally are more resistant to weathering and form higher rockier hills than the Deakin Volcanics.
Further south from Mount Arawang beyond the southern limit of our park we come to Neighbour Hill (to the west of Kambah) and a little further to the west Forster Hill (closer to the Murrumbidgee River). These two hills consist of porphyry very similar in chemical composition to the Laidlaw Volcanic Suite but intruded into the volcanics at depth in the earth's crust. The porphyries are Silurian in age and are some millions of years younger than the Laidlaw Volcanic Suite. Porphyries, because they were intruded at depth and unlike volcanic lavas and ashes, had some time to cool. However, they were not intruded at great depth and as a consequence the minerals they contain have not all had the opportunity to crystallise fully. Consequently, we find that some minerals such as quartz and feldspar have developed as large crystals (up to 2 cm) while the rest of the rock forms a very fine matrix.
Abell, Robert S., 1992, Geology of the Canberra 1:100 000 Sheet area, New South Wales and Australian Capital Territory Bureau of Mineral Resources Geology and Geophysics, Bulletin 233, 116p., plus map sheet.
Struckmeyer, H.I.M. and Totterdell, J.M., ed., 1992, Australia, Evolution of a Continent. Bureau of Mineral Resources Geology and Geophysics, Canberra, ACT.