Lake Gilles was once a shallow ephemeral freshwater lake. It formed in the Miocene epoch between 24 and 5 million years ago when the climate was wetter than it is today. Faulting of the underlying bedrock created a closed low-lying basin that collected runoff from the surrounding area.
During the Quaternary Period, from around 2 million years ago, the climate in this area became more arid. The rate of evaporation exceeeded the rate of flow of water into the lake and Lake Gilles developed into a salt lake. Salt lakes have a high concentration of salts, often more saline than sea water.
The water that drains into the lake is a combination of rainwater and ground water, which contains salts weathered from surrounding bedrock. Common ions in saline lakes are sodium, calcium, magnesium, carbonate, chloride and sulphate. With increased evaporation, these ions combine and form calcium carbonate, gypsum and halite. Lake Gilles produces some of the most unusual forms of gypsum in the world, including hour-glass and arrow-head shaped crystals.
The dunes around the lake formed when gypsum from the dry lake surface was broken down into tiny grains and blown by the wind and deposited on the lake margins. Some of the gypsum has been mined and used for agricultural purposes as a soil conditioner.
At GZ you will be standing on a gypsum dune. The surface gypsum has been hardened and coloured due to the effects of weather, plant growth (including mosses and lichens) and staining by dust and minerals from nearby soils. However, examples of the underlying gypsum that was blown by the wind to form the dunes can be easily seen where burrowing insects and other small animals have brought it to the surface.
To log this earthcache...
Please send me a message or email addressing each of the following questions and tasks based on your observations at GZ and the information supplied in the Cache Description above:
- Examine some of the underlying gypsum where it has been brought to the surface. Describe its colour, texture and how it feels.
- There are no larger gypsum crystals (such as the hour-glass and arrow-head shaped crystals) at GZ. Why do you think that is and where would you expect to find them?
- Take a photo of the lake from GZ, including either yourself or something showing your caching name and include it in your log.
You may log your visit once you have submitted your answers to me. I will respond confirming permission to log, or seeking clarification if I feel that you have not properly answered the questions. Logs with no answers sent or photo included will be deleted.
Please do not post your answers or spoiler photographs in your log.
Enjoy your visit to Lake Gilles!