Little Grand Canyon EarthCache

Porcupine Gorge is an ancient and special place. There are very few sites which offer such a clear insight into Queensland’s geological development. The gorge extends 25 kilometres along Porcupine Creek and 120 metres down into the earth to reveal almost 200 million years of geological history.

Porcupine Gorge is the result of natural erosive powers of water. Millions of years ago thick layers of sand were deposited at the bottom of an extensive inland sea covering what is now this part of Queensland. In time, the pressure from overlying material converted the sand into sandstone.

Sometime in the prehistoric past, forces in the earth’s crust uplifted this region above the lower country to the south. Between 6 and 2 million years ago it was then covered by sheets of lava that erupted from small volcanoes. The lava hardened to basalt which formed a hard cap over the elevated land surface.

Eventually, Porcupine Creek began to erode through the basalt and, upon finding softer sedimentary rocks beneath, it rapidly (in geological terms) cut down to its present level. The hard basalt cap prevented its sandstone sides from eroding as quickly, creating the vertical walls of the gorge. At the bottom of the gorge the sandstone that has been exposed and shaped by water and wind over this time is smaller and rounder. Today the creek is still eroding the sandstone and has formed a ‘gutter’ in its wake.

There are several visible coloured strata of sedimentary rock in the gorge walls portraying the changes that occurred in the landscape those hundreds of millions of years ago. These were deposited by two separate river systems. The Wallumbilla Formation extends under much of Northern Queensland and is known to contain marine fossils. The Gorge lookout where you are standing is on this layer.

If you choose to, you can walk the 2.4km to the floor of the gorge. However, it is not necessary to complete this Earthcache. Along the walk the Earth’s ancient past is exposed. There are many uncovered layers of sedimentary rock such as the isolated monolith known as the Pyramid. This spectacular rock formation is composed of layer upon layer of flat-bedded sandstone. It was eroded by Porcupine Creek to produce equally inclined sides, shaped like a pyramid. It still trickles through the gorge today continuing its age-old process of erosion.

At the base of the pyramid is purple-white Warang Sandstone which is 240-235 million years old and has been sculptured by the water into attractive fluted patterns. Further upstream, are exposed Boonderoo beds which are sediments consisting of debris from glaciers about 300-290 million years ago. There are also schists and gneisses which may be as old as 600 million years.

During the considerable gaps in geological time between the various layers in Porcupine Gorge new rocks were not laid down.

Porcupine Gorge: Kennedy Developmental Road (73 kms north of Hughenden)
Cost: Entry to the park is free, however fees apply for camping
Hours: Open 24 hours per day Contact: Information, access and maps available at www.epa.qld.gov.au/parks_and_forests/find_a_park_or_forest/

You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted. The answers can be found by observing the landscape and reading the information boards. Please do not post your answers in your log.

Q1. Which sandstone layer was laid down first?

Q2. What is the name of the red-brown layer?

Q3. What colour is the Wallumbilla Formation and how old is it?

Q4. What kind of rocks are schists and gneisses?

Q5. What is a monolith?

Q6. Porcupine Creek is a tributary of which river?

Q7. Look at the gorge from the lookout. What coloured layers can you see from top to bottom?

It would be nice to see a photo of you with your favourite part of the gorge in the background. Please post one if you wish.