This small rocky beach on the margin of North West Bay is a natural laboratory for observing Permian sedimentary geology and the way water energy shapes resistant rocks into distinctive beach pebbles. The standout feature here is the abundance of white to pale grey chert pebbles, which have been weathered, rounded, and concentrated by long‑term weathering processes.
LOGGING TASKS
To claim this EarthCache, please answer the following questions based on your observations at the posted coordinates:
1. Pebble Size
Estimate the typical size of the chert pebbles you see along the beach. Are most of them closer to:
: marble-sized (1–3 cm)
: golf-ball sized (3–5 cm)
: tennis ball sized (5-10 cm)
: larger than 10 cm?
2. Pebble Shape
Examine several pebbles. Would you describe them mostly as:
: angular
: sub-rounded
: well-rounded
Explain what this tells you about how long the stones have been moved by water and wave action.
3. Colour Variation
Look carefully at the pebbles around you. List at least two different colours or shades you can observe in the chert.
3. Photo Required
Include a photo of you or something to identify you at the site with your log
Much of the shoreline around Lake Pedder is known for its wild landscapes, dark water and rugged mountains. But at this location you’ll find something unexpected, a shoreline covered in pale, smooth pebbles that almost look like fragments of porcelain. These stones are chert, a hard sedimentary rock made mostly of microcrystalline quartz. Over time, waves and weather have rounded and polished these fragments into the small stones scattered along the beach. Standing here you are seeing the result of millions of years of geological processes, from ancient seabeds to mountain uplift, followed by erosion and wave action that has shaped the shoreline you see today.
Chert typically forms on the floor of ancient oceans. Tiny marine organisms such as radiolarians and sponges have skeletons made of silica. When these organisms die, their silica-rich remains settle to the seabed. Over long periods, this silica accumulates and is compressed into layers of chert. Later geological forces lifted these rocks above sea level during the formation of Tasmania’s rugged southwest mountain ranges. Erosion slowly broke down these rocks, releasing fragments that rivers and gravity carried toward valleys. Eventually, these fragments found their way into what is now Southwest National Park and the basin that became Lake Pedder.
The modern lake you see today is much larger than the original natural lake. In the 1970s the area was flooded as part of the hydroelectric development by Hydro Tasmania. While the landscape changed dramatically, today, waves along the shoreline continue to sort, round and polish these durable stones, creating the striking white pebble beach you see here . Hard rocks like chert resist erosion and gradually accumulate along beaches where softer rocks break down or wash away. This process is called wave sorting, the same natural mechanism that forms many pebble beaches around the world.
If you pick up a pebble here (please return it afterwards), you’ll notice several characteristics: It is very hard stone with smooth, rounded edges. It is often white, grey, or slightly translucent and fine-grained with no visible crystals. These features are typical of chert. The smoothness comes from abrasion, where pebbles repeatedly collide with each other as waves move them back and forth across the shoreline. Over time sharp fragments become the rounded pebbles you see today.
This EarthCache invites you to explore these pebbles and discover the geological journey that brought them from deep ocean sediments to the edge of one of Tasmania’s most iconic wilderness lakes.