Coastal headlands - a natural history

21 Nov 2011 1:35 PMSusan Guthrie
Coastal headlands - a natural history

PART 1: Point Danger and Snapper Rocks. 

 

During the 2011 Quicksilver Pro earlier this year, one of the commentators continually referred to the sharp rocks at Snapper Point as coming from the 23 million year old Tweed Volcano. Ian Black, nature enthusiast from Geo-Nature Walks and Tours, hopes you find the correct facts much more interesting.

 

Just like Currumbin, Elephant and Flat rock, Snapper rocks are made up of steeply inclined well-bedded argillite of the Neranleigh-Fernvale beds (NF beds) and date back to 240-290 million years ago (MYA).

 

Around 290-370 MYA, the coastline was west of Toowoomba and where the Gold Coast now sits there was a deep ocean trench. Over millions of years, eroded sediments washed into the trench and continued to build up in great depths. At 240-290 MYA, the ocean plate subducted under the continental plate and with immense heat and pressure, the sediments were scraped off the ocean floor, crumpled and folded to form a steeply inclined strata of metamorphosed rocks. Eventually the NF beds were thrust up above the sea level to form high mountainous terrain.

 

By 23 MYA, a number of deep valleys had worn their way down thru the meta-sediments to the coastline and the first flows of basalt from the Tweed Volcano filled the ancient valleys up. Now that the valleys were full of basalt, rainwater could not flow down them anymore and the older NF beds either side of the valley started to erode. One of these deep valleys was at Point Danger and the thick basalt flow can easily be seen from the picnic area at the north end of the Duranbah car park (see main photo).

 

The old valley’s width seems to finish at Froggy’s to the north and Lovers to the south. Over the past 23 million years, the sides of the valley have mostly eroded away leaving only a small hill of NF beds at Snapper and either side of the flow. The council have placed basalt boulders around the walkways at Snapper to help stop erosion of the hill and for beautification.

 

Originally Pt Danger rounded off into the ocean until it was quarried from mid-1890 -1910 to supply rocks for the original Tweed River training walls (photo 2). The sharp rocks mentioned earlier are argillite, steeply inclined fine silts, mud hardened and slightly recrystallised having been closely fractured (like a poor quality slate) and can be sharp underfoot. They have resisted erosion as the headland has worn back to its present position (photo 3).

 

Interested in learning more about the area?  Contact Ian Black of Geo-Nature Walks and Tours for a copy of the latest full colour edition ‘Rocks and Landscapes of the Gold Coast and Hinterland’ by Warwick Willmott of the Geological Society of Australia, Queensland Division. Available for $15 plus postage.

 

 

Or better still, join one of Geo-Nature’s guided walks for a fascinating and informative introduction to the Gold Coast and surrounds.

 

Contact us today!