Lane Cove’s Hawkesbury Sandstone: A Window into Sydney’s Geological Past
Location & Access
This EarthCache is located within a National Park. While entry by car requires a fee, access on foot is free. The geological site is approximately 1 km along a comfortable, tarred road from the south side park entrance. To assist visitors, recommended waypoints for parking access have been included. GZ is on the main roadway and easily spotted.
Geological Significance: Visible Hawkesbury Sandstone
When looking from any vantage point of Sydney Harbour, it is easy to overlook the geological history of what this would have looked at 200 years ago. The fact is that you are looking at the lower portion of a valley. This area's valleys were a scene of rolling hills, greenery and virgin forest. The geological history is hidden by the buildings and bridges etc.
Therefore, this EarthCache is to take visitors back in time to Lane Cove where the same geological features have been preserved. The dominant rock in both places is Hawkesbury Sandstone.
The formations at GZ are not just another example of exposed Hawkesbury Sandstone; they reveal a preserved part of Sydney’s geological history. Unlike more common sandstone outcrops, this location showcases clear stratification and unique mineral features that provide clues about ancient environments, in a rainforest setting.
The Hawkesbury Sandstone is a significant geological formation within the Sydney Basin, varying in thickness from approximately 34 meters in the lower Blue Mountains to around 240 meters near the Hawkesbury River. This vast rock unit spans roughly 12,500 square kilometres and is primarily composed of quartz-rich sandstone, with minor shale content making up about 5% of its composition. One of its defining features is the abundance of cross-bedded layers, which provide valuable insights into ancient depositional environments.
Cross Bedding & Sedimentary Processes
Cross bedding, a characteristic seen throughout the Hawkesbury Sandstone, is a structural feature formed by sediment transport in dynamic environments, such as ancient river systems. As water currents moved sand across the landscape, the grains settled into angled layers, creating a distinctive pattern of thin, intersecting laminae. These layers, deposited at an angle rather than parallel to the main bedding surface, reflect the shifting flow of ancient waterways that once meandered through the region.
Additionally, the sandstone exhibits lens-like beds that appear discontinuous, a result of variations in sediment supply and deposition over time. Many of its quartz grains display a reflective sheen due to silica overgrowth, contributing to the rock’s subtle sparkle in certain exposures.
The Hawkesbury Sandstone is a Triassic-period sedimentary rock, formed from quartz-rich sands transported by a vast river system that once existed across the Sydney Basin. These sands accumulated over time, and with the pressure of overlying materials, lithified into solid rock. The sandstone here records the dynamic flow and shifting deposition of these ancient waterways.
This site is particularly noteworthy because of its distinctive mineral influence, erosion patterns, and biological interactions, which together demonstrate how geological processes continue shaping the environment today.
Key Geological Features
-
Sedimentary Layering: At GZ, visitors can observe subtle and obvious variations in grain size and composition, indicating fluctuating flow rates in the prehistoric river system.
-
Iron Oxide Staining: The rich orange-brown hues seen in the rock result from iron oxides deposited during sedimentation, adding valuable insight into ancient chemical conditions.
-
Erosion Patterns: While sandstone is a resistant rock, wind, water, and biological activity contribute to ongoing weathering, creating visible cracks, ledges, and microhabitats.
-
Porosity & Plant Growth: The porous nature of the rock allows moisture to penetrate, fostering mosses, ferns, and lichens that thrive in small pockets of retained water.
Geocache Challenge
To successfully log this EarthCache, please complete the following observations at GZ:
-
Stratification Analysis – Describe the layering in the rock. What variations in texture and colour do you notice?
-
Erosion Activity – Identify visible signs of erosion. What natural forces are shaping the rock today?
-
Hardness Test – Using a simple scratch test (a fingernail or a coin), determine whether the sandstone crumbles easily or remains solid.
Additional Requirements
-
Photo Submission with LOG: A picture taken at GZ showing yourself or an identifying object (e.g., geocaching logo). Please ensure that the photo does not reveal specific geological formations, as per EarthCache guidelines.
Educational Sources & References
To support the geological lesson, the following resources were consulted:
-
A guide to Australian rocks, fossils and landscapes by Russell Ferrett
-
Geological Survey of New South Wales– Provides detailed information on Sydney Basin formations.
-
"Hawkesbury Sandstone – Its Origin and Characteristics," Journal of Australian Geology, Vol. 22.
-
Official research from the Geological Society of Australia.
Final Notes
This EarthCache highlights an exceptional site where visitors can directly engage with Sydney’s prehistoric landscapes and the processes that continue shaping them today. By participating, geocachers not only complete a challenge but also deepen their appreciation of geological science.