Black Mountain

During the last ice age, huge ice sheets covered North America and Europe. However, in Australia there were only small glaciers on the Australian mainland positioned around Mt Kosciuszko. Throughout the mountains of Australia there is evidence of much colder conditions in the form of periglacial deposits. These are landforms that develop due to the action of frost, ice and snow on the landscape, in the absence of glaciers. There is also evidence of slope instability during this time. Alluvial fans and bajadas developed along the hills of the Southern Tablelands. In places along rivers and lakes, dunes blew up. These observations point to a very different landscape to present.

To calculate the temperature change during the last ice age, I led a study to determine the extent of the periglacial environment on the Southern Tablelands¹. A wide range of different estimates have been published ranging from 6 to 14 °C colder than today. Previous studies identified landsforms characteristic of the periglacial margin at the altitude of Canberra. We focussed on Black Mountain in Canberra and a few other key sites around the region.

The 3D model above was made from aerial photos of Canberra taken in the early 1990s when the study began. It shows the steep upper slopes with thin soils and the broad flanks. The southwest and eastern sides of the mountain have alluvial fans. The fans are no longer active and the slopes were covered in trees until British colonisation. I mapped the geomorphology of the mountain and identified the location of the fans through exposures in erosional gullies. I also found a number of surface deposits of scree. These are angular blocks that have accumulated on the surface as a result of frost cracking. We chose seven sites on the southern side of the mountain to date the formation of the alluvial deposits and the scree. These kinds of deposits are difficult to date, so we chose three different approaches; radiocarbon, optically stimulated luminescence and profile dating with cosmogenic nuclides.

Somewhat surprisingly, there is almost no organic material in the sections that could be used for radiocarbon dating. This usually indicates the environment of deposition was dry. The only section to have charcoal was in the southwest fan and it was at the limit of radiocarbon dating (>50,000 year old). The optical dating revealed a much broader range of depositional ages than expected. The top layers of the alluvial fans and valley fills were deposited between 66,000 and 13,000 years ago, during the coldest phase of the last global glaciation. The profile dating showed that the fan on the eastern side of the mountain is at least a quarter of a million years old. A buried soil in the profile suggests that the history of the fan extends back at least half a million years.

To better understand the climate conditions, we modelled two different environmental variables. First we modelled how snow would be redistributed over the mountain by the prevailing wind (Figure 1) and second, we modelled shading across the mountain during winter (Figure 2). The modelling showed that snow would accumulate on the eastern slopes of the mountains in the lees. Melting of the snow probably assisted erosion of soil on these slopes, which accumulated in the gullies and on the fans. The radiation modelling shows that the screes are all in shaded areas. Low temperatures were vital to the formation of these.

The elevation of the Black Mountain scree matches the elevation of several other sites on the Southern Tablelands. Today, the only conditions cold enough to form these kinds of deposits are found up at Mt Kosciuszko. Therefore, the temperature difference in Winter was at least 8 degrees colder. Cold temperatures will have some important affects on the landscape apart frost cracking of rock and erosion of soil. First, any snow that falls will last longer in the environment. When it melts in Spring, there is much more water to contribute to the rivers. Second, lower temperature means lower evaporation, so more water remains in the soil, rivers and lakes. Last, colder temperatures mean fewer plants can survive, especially frost prone ones, resulting in barer slopes. This means that runoff is higher into the rivers. These are the likely reasons why rivers were able to paradoxically carry more water out into the desert and fill lakes, like Lake Mungo.

1. Barrows, T.T., Mills, S.C., Fitzsimmons, K., Wasson, R., Galloway, R., 2021. Low-altitude periglacial activity in southeastern Australia during the late Pleistocene. Quaternary Research, 1-22. 10.1017/qua.2021.72