“It will cost the Main Roads Commission many thousands ofWhy are there no detailed records of the most violent landslide in Australian recorded history? Despite being in living memory and cutting the highway to Port Douglas for a few weeks, finding a photograph of this event or even a map of where it occurred seems to be impossible. I have been trying to piece together what happened for a few years now and the story just gets bigger.
pounds to repair a six-mile stretch of the Cook Highway between
Buchan and Simpson's Points, following huge landslides caused
by a torrential downpour lasting nearly five hours.
Almost unbelievable quantities of earth and debris were swept
from the mountain-side down on to the roadway and over the
precipice into the sea. Gigantic trees were uprooted and ground to
pulp, and boulders as high as 10 feet hurled into the Pacific- as if
they were marbles.
Millions of gallons of water cascaded down the mountains into
the sea, gouging huge ravines and making swiftly running streams
in the thousands of tons of earth and rubble left on the road in the
wake of the slides.”
Near Cairns there are a number of places which are prone to these massive events, which could be up to 1000 times larger than the tragic Thredbo landslide. These events could cause serious loss of life and property and there needs to be less complacence about this issue.
Approximately 2 km north of Ellis Beach are a few pretty sandy beaches with boulder headlands at each end. A further 1.5 km north there is a boulder beach that is 3.5 km long. The origin of the boulders on this mostly ignored stretch of coast is the subject of this post. The native bedrock of this coastline is a slate-like metamorphic stone whereas the boulders are granite so it is clear that the boulders came from somewhere else. These boulders provide a means of tracing the debris flows back to their origins.
|A boulder beach near Ellis Beach in Far North Queensland (Click to enlarge)|
|Sand polished boulders near low tide level|
|Aerial view of Simpsons Point|
|Simpsons Point from the side, showing boulders pushed into the sea by a debris flow|
While most debris fans have dry rainforest, the debris fan in the catchment explored in this post had a glade of cycads that was more than 100 metres across. Cycads are at their best in rocky ground.
|The glade of cycads|
|Herbs that spring up from bulbs during the wet seasons fill spaces between the cycads|
|Ironbark woodlands cover metamorphic hills beside the old debris flows|
|A naturally bare patch showing metamorphic rock fracturing and flaking|
|Solid metamorphic rock lies below the surface|
|Cardwell lilies fill the understorey on the new forest in the gully|
|Red earth and stone between the tree roots show the original character of the debris flow|
|A contact zone slip plane with a layer of weathered metamorphic rock resting on top|
|A big rock that almost kept rolling|
|A wedge of soil on a slip plane. The slope in the foreground probably shed its load.|
When the ground does let go, the one thing that is certain is that the debris flow hurtles down the gullies. Beside the small creek I followed, the debris flow averaged 30 m wide and formed an elevated inclined plain. It is likely that nearly all the trees present in the gully would have been ripped from the ground and carried away. Occasionally, when a few large trees formed a row across the gully, they were able stand against the debris flow. These trees by virtue of their being there to halt part of the debris flow provide indirect evidence that the flow occurred decades ago, not hundreds of years ago. In many places the sides of the gully were scoured back to bedrock. The evidence shown here is repeated in most of the creeks on this section of coastline. On some of the larger and most damaged creeks, the vegetation has not recovered. When the trees were destroyed, tall exotic grasses and lantana moved in to form a blanket of weeds that have suppressed the regeneration of forest.
|The sloping surface of the old debris flow occupies the middle of the gully|
|A few trees stood against the onslaught and built a wall of rocks|
|The tree on the right has roots at two levels showing that it has seen 2 debris flows.|