Geologists like to use big words to describe rocks and landforms. Geomorphology is simply the study (-ology) of land forms (morph) such as mountains, coastal plains, lake and shorelines which are effected by the underlying geology (geo-) e.g. rock, gravel, sand and mud. Geology is not an exact science and even amongst geologists they will come up with different answers for the same location. So try your best.
Logging Tasks
To claim this EarthCache, please complete the following at the posted coordinates and walk towards the lake edge ot the south. This Earthcache can be completed anywhere along the northern lake edge.:
- Shore material. At the water’s edge, which material is dominant?
(A) clean sand (gritty between the fingers)
(B) clay/silt (mud)
(C) organic peat (black swampy material)
(D) gravel/rock (small pebbles to large pieces of rock)
Provide one short observation (e.g., “A — loose white sand, grains visible”).
- Landform (geomorphic) setting. Which best describes Black Swan Lake?
(A) interdunal wetland
(B) estuarine lagoon
(C) karst sinkhole
Give a one-line justification for your choice.
- Water level evidence. Look for signs such as a wet/dry line,or stranded leaf litter/ branches twigs. Is today’s lake level higher, the same, or lower than the recent high-water line? Give one observable feature to support your answer. Note: at times the water can be over the walking tracks and at others it can be almost dry.
- Photo requirement.
• Take a photo of yourself or a personal item (GPS, notebook, or hand signal) with Black Swan Lake in the background.
Geological Background
Black Swan Lake lies on the Swan Coastal Plain, a low sandy plain formed during the Quaternary period by coastal dune building, wind action, and fluctuating sea levels. The plain is made up of a series of dune ridges separated by swales (depressions in between the dunes). Many of these swales host wetlands, where the shallow groundwater table intersects the land surface.
This lake is a good example of an interdunal wetland. The surrounding ridges are built from well-sorted limestone and quartz-rich sands blown inland by coastal winds. These sands dominate the shoreline material (Question 1). At the lake margins, organic matter can also accumulate, forming darker peat-rich zones in sheltered spots.
The landform (geomorphic) setting of the lake (Question 2) reflects its position in a swale between dune ridges. Unlike estuarine lakes connected to the sea e.g the lakes in Mandurah or karst lakes, Black Swan Lake has no tidal connection to the sea and no underlying limestone sinkhole. Its form is directly controlled by the sandy coastal landforms of the Swan Coastal Plain.
The water level (Question 3) varies with rainfall and seasonal groundwater changes. As levels rise and fall, they leave visible evidence along the banks, such as algae lines, stranded leaf litter, or exposed roots. These shoreline features allow visitors to interpret recent hydrological changes and link them to the geomorphology of the wetland system. The water level in lakes can vary dramatically with paths flooded at times or the lake being alomst dry.
Nearby wetlands, such as Paganoni Swamp, show similar processes and are part of a wider network of conservation wetlands. Together, these sites highlight the interaction between coastal dune formation, groundwater flow, and wetland ecology — key features of Swan Coastal Plain geomorphology.
Logging Note
You can log the cache immediately. Please send your answers within 14 days. Logs without answers will be discreetly deleted.