Hundreds of millions of years ago two vast land masses crashed together. As they collided the rocks crumpled and folded shaping this landscape. The edge of one of the landmasses was pushed upwards forming the Hamersley mountain range.
The Hamersley is part of a landmass 3500 million years old. The oldest rocks remaining today, the granite-greenstones of the Pilbara, were formed from the outpourings of huge volcanoes and sediment eroded from even older rocks.
About 2700 million years ago the continental plate that contained the Pilbara rifted apart, weakening the crust and allowing basalt lava to flood the land. Eventually, the continent split into two, the volcanic eruptions ceased and a shallow ocean formed over this new land.
The Pilbara is commonly referred to as iron-ore country. Deposits of banded iron-formation in the Pilbara are among the most extensive in the world. The bands visible in the rocks and gorges in this area are dark grey, red and white. Between 135 - 65 million years ago the iron accumulated under a soft clay layer near the ground surface.
During the Cretaceous Period the clay was eroded away exposing this iron-rich layer. Around the same time, some of the iron which had dissolved from the deeply weathered bedrock was then deposited in river channels, swamps and marshes on the valley floors. The iron-ore being mined in areas outside Karijini National Park is from such valley floor deposits.
The banded iron-formations and hard caprocks contribute to the distinctive character of this landscape. They are generally resistant to erosion. However, rainfall in the area, while variable and spasmodic can be powerfully erosive.
Fast-flowing water and abrasive rock debris erodes the banded iron-formation along weaknesses which results in the softer layers of rock being worn away. This undercuts the rocks creating large overhangs which eventually collapse. Over time, the flowing water removes the broken rocks and debris, leaving behind terraces or platforms.
Creeks have continued to cut through the ranges over millions of years forming spectacular gorges in the area. Water seeping out of fractured and permeable rocks replenish permanent pools during the dry months. Erosion is continuing today and will continue into the future.
At Oxer lookout - refer to Q4 below.
Over 2500 million years ago where you are now standing was the sea floor. Layers of silica and iron oxide with silica built up over time, squeezing out the water to form tough, well-bedded rock.
Colliding continental plates caused these rocks to buckle and develop numerous vertical cracks before being lifted up to form dry land. Erosion over millions of years has sculptured the rocks into the present landscape you see at Oxer lookout. Here there is evidence of the geological formation of some of the Hamersley gorges.
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 answer to Q4 can be found through observation at Oxer lookout. Supporting information for the other questions can be found through the visitor centre and a bit of research. Please do not post your answers after you have received permission to log.
Q1. Name the two landmasses which collided to form the Hamersley Range.
Q2. What is the term which refers to cracks in the rocks where gorges have formed?
Q3. What is meant by permeable?
Q4. Compare the two gorges at opposite sides of the Oxer lookout platform. What geological differences do you observe between them?
Please post a photo of an aspect of remarkable geology from the Hamersley Range with your log if you wish. If you are in the photo that would be even better.
We hope you have enjoyed learning about the geological development of the Hamersley Range at this Earthcache.