The Sculptures of Sydney Harbour EarthCache

LOGGING TASKS:

To log this EarthCache, please answer these questions and send the answers to us using the Geocaching Messenger system.

1. Compare the exposed rocks at the posted coordinates with the rocks along the ocean at Stage 2. Describe the texture and colours of the rocks at each stage. Are these both the same kind of rock? What rock is it?

2. What is the primary difference between weathering and erosion?

3. Based on the explanations in the description, why do these rocks look different? What processes have occured at the given coordinates and what processes have occurred at Stage 2? Consider specifically how different environmental factors may have influenced what you now observe.

4. Include with your Found It log, a picture of yourself OR something that has your Geocaching name/handle written on it, in front of the Sandstone Galleries at the given waypoint.

The Rock:

The rock you are standing near is known as Sydney Sandstone (specifically part of the Hawkesbury Sandstone group). This rock is approximately 230 million years old, dating back to the Triassic Period. Sandstone is a sedimentary rock and is a softer rock than the igneous rocks many of the buildings and pavements in Sydney are constructed from.  At the EarthCache given coordinates, we want you to face away from the water to look at the large wave-like formations of rock here. In our experience of similar formations on the West Coast of British Columbia, we call these "Galleries." At Stage 2 you will see some more rock below the pavements and grassy area, in the tidal zone. We want you to note the differences in appearance of the rock - there should be no need to go down into the tidal area of Stage 2.

How was it formed?

In prehistoric times, a massive river system, larger than the Amazon, was flowing from the south (what is now Antarctica) into a giant basin. This river carried immense amounts of sand. Over millions of years, these sand layers were buried and compressed. Sandstone is made up of three main parts; grains, matrix, and cement. Grains are the parts visible to the naked eye - the grains of sand you can see in the stone. The matrix is the very fine, microscopic particles (like silt or clay) that were deposited at the same time as the sand and filled the spaces between the grains. The cement is a chemical "glue" (like silica, calcium, or iron oxide) that hardens over time, turning the loose pile of sand and matrix into a solid block of stone with the application of time and pressure. 

One of the things special about Sydney Sandstone is its cement. The sand grains are held together primarily by silica and iron oxides, which will erode, or wear away, differently, depending on the environmental factors affecting it over time. 

Weathering vs Erosion

In this lesson we're using the terms weathering and erosion. An imporant difference between the two is that weathering is the various processes for breaking down rock. Erosion is the movement of the broken rock material. This lesson is intended to help us understand how the same rock, in very close proximity, can look very different depending on whether it is more subject to weathering or erosion, and which types might be at play.

Weathering can be physical (mechanical) where the rock is broken down without changing it's primary composition, including by water freezing and thawing in the cracks of the rocks, the rock expanding in the heat of the sun, or the growth of salt crystals into the rock forcing it apart. Chemical weathering is when some chemical process changes the composition of the rock, weakening it and causing it fall apart. This can be due to oxidation (such as iron oxides rusting), or acid rain disolving limestone. There is also biological weathering where plants or animals cause the rock to break down through the growth of roots, or mosses and lichens.

Erosion happens when rock is broken down and transported (moved away).  Erosion can take place from water (e.g.: wave action, river currents) constantly hitting rock and moving the resulting broken bits away. Wind can work in a similar manner, and is often seen in dry regions where it blows away sand. Where glaciers have been present, you wil see the results of ice or glacial erosion as the enormous pressure of the glaciers have scraped away rock. Lastly, gravity is a type of erosion where the force of gravity moves rock that has weakened downwards.

The results of both of these geological processes (weathering and erosion) can be observed at the waypoints of this EarthCache, and result in the same rock having a very different appearance.

The Weathering of the Sandstone 

At the given EarthCache coordinates, observe the wave-like shapes and hollowed-out "galleries." These often aren't actually carved by the ocean waves at this height; they are carved by salt and wind over hundreds and thousands of years. This is due to processes called chemical and physical weathering, and wind erosion that erodes the rock over time by breaking down the matrix that holds the sandstone together. Here is a simplified explanation of how this has occured.

1. Salt Spray: Saltwater from the harbour spray soaks into the porous sandstone.

2. Crystal Growth: As the water evaporates, salt crystals grow between the sand grains.

3. The "Pop": These crystals act like tiny wedges, pushing the sand grains apart.

4. Hollowing Out: Because some parts of the rock are slightly softer or have less "glue," they erode faster, leaving behind the harder outer shells and creating those beautiful, smooth, wave-like curves you see in the cliffs. Wind and gravity erode the rock, removing the debris, often not transporting it very far, but just enough to form these picturesque galleries. 

The "Galleries" at the given coordinates

At Stage 2, you see the sandstone has been affected very differently, resulting in a different appearance. Because this rock is in the tidal zone, it is constantly being "scrubbed" by the waves, and the salt water is impacting the matrix much more quickly. You will also notice circles in the top of the stone bedding, caused by broken rocks and debris being "circled" by the wave action resulting in small tidepools or "potholes." This mechanical weathering process is combined with water erosion where the ocean waves are not only breaking down the rock, but moving it away, resulting in the holes, gaps, and breaks you can see here.

The Comparison: Upper Gallery vs. Tidal Zone

Let's discuss the difference between the two observation spots.

• Given Coordinates/The "Galleries": These are shaped by chemical and physical weathering (salt crystallization), and subsequently affected by wind and gravity erosion. The shapes are smooth, rounded, and you can even see some spots that look like a honeycomb - sometimes refered to as tafoni.

• The Tidal Zone Rocks: These are also affected by physical and and chemical weathering, but the biggest difference is that they are being hit by water erosion in this tidal zone. The constant pounding by the tide and the debris carried by the waves makes these rocks flatter and more jagged, and the ocean waves and currents carry away the broken bits of rock.

With the information in the description plus your own observations at the waypoints, you should now be able to complete this EarthCache experience by conducting the observations requested and submitting your answers to us. Observe the stone appearance at both waypoints, consider how the stone is formed, the processes of weathering and erosion, and the different environmental factors at play in either location.

References:

• The Australian Museum – "The Sydney Basin":  Stratigraphy of the Hawkesbury Sandstone and its quartz-rich composition.

• Herbert, C. & Helby, R. (Eds.) – "A Guide to the Sydney Basin"

• Australian Coastal Society – "Honeycomb Weathering"

• Rodriguez-Navarro, C., et al. – "The Origin of Honeycomb Weathering"