Please park carefully so as not to obstruct resident access.
If you look across the valley from the GZ you will see the Dandenong Ranges which are the remains of an ancient volcano formed in the late Devonian period.
The size and shape of the mountain has changed over its 300 million years and is still changing, but that usually happens so slowly that we do not notice. One exception to that happened in July 1891 - the Great Montrose Landslide.
Many landslides (sometimes called a landslips or debris flows) occur after an earthquake, but not this one. After 3 days of torrential rain, earth and boulders were loosened on the upper, steep part of the mountain that you can see to your right, and they started to move and slide down the mountain.
Part of the landslip from https://collections.museumvictoria.com.au/items/1449178
The landslide traveled up to an estimated 40 km per hour, and spread to over 50 metres wide on the upper slopes. On the lower slopes it was more contained, following the existing gullies, and eventually traveled a distance of about 2 kilometres, following Swansea Road to your left towards what is now York Road. It is estimated that 60,000 tonnes of earth and rocks moved that day.
This figure is from: Moon, A.T., Olds, R.J., Wilson, R.A., and Burman, B.C. 1992. Debris from flow risk zoning at Montrose, Victoria. In Proceedings of the 6th International Symposium on Landslides. Vol. 2. Edited by D.H. Bell.
Some people thought that trees had been removed from the landslide source and that may have made the land more susceptible to a landslide.
Fortunately the area was sparsely populated in 1891, and although the land changed its appearance, only one house was lost and no one was killed, although two people had to be rescued. Since then the lower slopes of the mountain have become a residential area, and if the same landslide were to happen today there would be severe property damage and probably loss of life.
Studies of the debris zones and the geology of the area in the 1990’s led to the development of a model to try and predict the risks areas associated with landslides and landslips, and this “Montrose model” has been the used in many other studies worldwide. It has also been used in generating the Yarra Ranges Shire Erosion Management Overlay. New landslips are known to occur where previous landslips have happened
You can log the cache immediately. Please answer the following questions to complete this cache and message the answers to the CO.
- What two factors are thought to have caused this landslide?
- Roughly, how long might it have taken the landslide to travel to its farthest point?
- In what ways are studies of debris flows useful?
- What do you think might change the shape of the mountain in the future?
- Looking at the mountain in front of you, describe where you think the landslip started. Using the site plan above and reference points you can see will be helpful.
There is a longer description of the landslide in the first 2 references below. There is another earthcache dealing with the Dandenong Ranges that you might like at GC5K6A9.
I hope you enjoyed your visit. Thanks go to heymissjo for additional information.
References
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