This EarthCache lesson is designed to teach about the deposits of sediments around the Hawkesbury River.
The Hawkesbury River area is about 21,000 square kilometres.
The river is believed to have been formed around 15 million years ago during the Permian and Triassic Periods, 290 - 200 million years ago.
The flow from further upstream from this EarthCache travels through several geological regions, each with varying types of rocks in the immediate vicinity. These includes sedimentary rocks and igneous rocks in the form of outcrops.
The flow of the river results in alluvial deposits, which is made of loose clay, silt, sand or gravel. The Hawkesbury River has a wide, flat floodplain in many areas, which is created by the deposition of these sediments.
This particular area of the Hawkesbury in Windsor are extremely rich in alluvial deposits. These more rich sections are most commonly found in areas where the rivers flow is slower due to a variety of reasons.
(An image from a study by the NSW Government of the area on the alluvial deposits, 2021
source: https://majorprojects.planningportal.nsw.gov.au/)
Alluvial deposits have major benefits on rivers such as the Hawkesbury, including:
- Fertility: Alluvial deposits are usually very fertile due to the nutrients carried downstream by the river. This promotes growth of plants on the banks.
- Erosion control: Alluvial deposits help reduce erosion by stabilizing the riverbank and by preventing soil from being washed away. This helps protect nearby land from damage caused by floods and other natural events.
- Carbon sequestration: Sediments in alluvial deposits can also play a role in carbon sequestration. Alluvial deposits can store large amounts of organic matter which helps to mitigate the effects of climate change by reducing the amount of carbon dioxide in the atmosphere.
The alluvial deposits here are most commonly clay or silty sand. These deposits overlay siltstone, sandstone and bedrock which are part of the Wianamatta Group.
Logging requirements:
Visit the given coordinates, make some observations and answer the following questions:
Q1. Why do you think this area has more alluvial rich banks then other sections of the river have?
Q2. How does the shape of the river have an impact on the area being more alluvial rich?
Q3. Can you see any exposed sediments at GZ? Is it clay or silty sand?
Q4. Take a photo of you, your stamp or personal item at the given coordinates with the river in the background.
Please send answers via the Geocaching message centre or to my email, answers posted in the logs will be deleted.
You can log the find immediately, if answers are not supplied to CO within 14 days, your log will be deleted.
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