The aim of this Earthcache is to learn about the formation and features of Yanchep Lagoon.
Yanchep Lagoon is located around 50km north of Perth. It is an extremely popular swimming and snorkelling spot and well worth a visit. It boasts calm, crystal clear water, with waves averaging just over 1 metre along the beach.
The safest swimming spots are the southern lagoon beach and the south end of the main lagoon. Please note that swimmers should swim between the flags and be cautious of the strong current which flows out of the lagoon into deeper water. If you are caught in a rip, stay calm, raise your hand and await rescue. Fishing is not permitted between the red and yellow flags and spearfishing is prohibited.
What is a lagoon?
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as a reef, a barrier island or islands, a barrier peninsula, or an isthmus. Lagoons are commonly divided into coastal lagoons (or barrier lagoons) and atoll lagoons. Lagoons are common coastal features around many parts of the world. The word "lagoon" originates from the Italian word laguna, referring to the waters surrounding Venice.
Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until eventually only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons often contain some deep (>20 m) sections.
Coastal lagoons form along gently sloping coasts where barrier islands or reefs can develop offshore, and the sea-level is rising relative to the land along the shore (either because of an intrinsic rise in sea-level, or subsidence of the land along the coast). Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres. Due to the gentle slope of the coast, coastal lagoons are shallow. A relative drop in sea level may leave a lagoon largely dry, while a rise in sea level may let the sea breach or destroy barrier islands, and leave reefs too deep underwater to protect the lagoon. Coastal lagoons are young and dynamic, and may be short-lived in geological terms. Coastal lagoons are common, occurring along nearly 15 percent of the world's shorelines.
Coastal lagoons are usually connected to the open ocean by inlets between barrier islands. The number and size of the inlets, precipitation, evaporation, and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, and high evaporation rates may become highly saline. Lagoons with no connection to the open ocean and significant inflow of fresh water may be entirely fresh. On the other hand, lagoons with many wide inlets have strong tidal currents. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, and from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may occasionally be deposited in a lagoon when storm waves overwash barrier islands.
Formation of Yanchep lagoon
Yanchep Lagoon is a "perched beach" coastal lagoon formed by a natural barrier of Tamala Limestone. Unlike the atolls or barrier island lagoons mentioned earlier, its formation is tied to the unique geology of Western Australia's coastline.
1. Geological Foundation
The lagoon is part of the Tamala Limestone formation, a series of Pleistocene-aged ridges made of aeolianite (fossilised sand dunes). These ridges run parallel to the shore and were formed over hundreds of thousands of years through the accumulation and cementation of wind-blown calcium-rich sand.
2. Physical Formation
- The Reef Barrier: The lagoon itself was created as the ocean eroded these limestone ridges. A low-crested reef—essentially a submerged or semi-emergent section of this ancient limestone—sits roughly 20 metres offshore.
- Enclosure: This reef acts as a natural breakwater, partially enclosing a shallow body of water and protecting it from the full force of the Southern Ocean's waves.
- Sediment Trapping: The reef and associated headlands trap sand, creating a "perched" beach environment where the sand sits atop the rock structure.
3. Modern Dynamics
The lagoon's shape and depth are constantly changing due to seasonal sediment transport:
- Summer: Strong sea breezes cause sand to build up, sometimes overtopping the reef and partially infilling the lagoon.
- Winter: Powerful storms and northwesterly winds often erode this sand, widening and deepening the lagoon.
Because of its vulnerability to these seasonal changes, the local government frequently performs beach renourishment (replacing lost sand) to maintain the lagoon's safe swimming areas.
Tasks
To log this cache, you will need to message me your answers to the following questions:
- What rock formation can you see in the cliffs and on the ground at the southern end of the beach?
- Describe the features of these rocks (colour, aspect, position etc)
- Based on the description above, what makes Yanchep lagoon fairly unique?
- Please post a photo of yourself or your geocaching name at GZ.
Please send me your answers using the geocaching message feature and I will contact you if I have any issues with them. You do not need to wait for permission to log this Earthcache.
Sources:
https://www.westernaustralia.com/au/attraction/yanchep-lagoon/5e6b40c87a82947726760112
https://en.wikipedia.org/wiki/Lagoon
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