Birridas – Gypsum Lagoons EarthCache

This earthcache will you require you to visit 3 waypoints and thus will take a little longer to complete than other earthcaches. If you are not prepared to stop at the 3 different locations then please don’t attempt this earthcache. All 3 waypoints can easily be done as you make your way out to Monkey Mia.

On the Peron Peninsula of Western Australia lies the World Heritage area of Shark Bay. This 2.2 million hectare area of beauty is littered with countless natural features. One of the visual phenoniums which is often overlooked by many is the birridas (dry lagoons) that cover this area. These gypsum lagoons which number over 100 support specially-adapted plant and animal species.  

Creation
Many years ago, when sea levels were much higher than they are today, birridas were landlocked saline lakes between sand dunes. A drop in sea level dried the lakes up creating the salty hollows of today. Birridas range in size from small, circular or oval depressions about 100m in diameter, to elongated or irregular depressions between the sand dunes that are several kilometres long.

When the birridas were lakes, sulphate of lime (calcium sulphate dihydroxide) was deposited onto the lake floor. When the lakes dried up the sulphate of lime evaporated and became loose, powdery gypsum. In the past, some of these gypsum deposits were mined.

Birridas commonly consist of a central, raised platform ringed by a moat-like depression. The central section corresponds to the level of the water table during the late Pleistocene Period. The moat was formed when fresh groundwater seeped from the surrounding dunes and dissolved the gypsum on the periphery of the central platform. Today, during very high winter tides, when the groundwater level is raised, some moats fill with water to a shallow level.

Little Lagoon
Little Lagoon is almost perfectly circular in shape, the lagoon was once a land-locked birridas that was inundated by the sea several thousand years ago. An important nursery for several fish species, the lagoon is now a marine recreation zone.

Big Lagoon
Big Lagoon is an example of what was once a land-locked saline lake that is now a shallow inland bay. Rising sea-levels in recent geological times inundated the small lake system creating this attractive turquoise lagoon. An important fish nursery, Big Lagoon’s northern waters are a sanctuary zone. Exploring the lagoon by canoe or sea-kayak is a great way to get up close and personal to this natural gem.

To claim a find on this earthcache
You will need to visit 3 waypoints where you will obtain the answers to the following questions.
At the posted coordinates (GZ) you will see some information boards.

1. What is the first timeline period mentioned and what happened at that time?" AND "what connects the two bodies of water?
2. Name two of the bird types that frequent these Birridas

At waypoint 2 S25° 53.231 E113° 33.650 you will be standing on a Birridas. This is a great example of a moat-like Birridas. Look towards the middle of the Birridas and you can clearly see the inner island.

1. Approx’ how much higher is the raised section of the lake.
2. As a Birridas is formed by receding sea levels, what is the height above sea level of this Birridas? Keep in mind this use to be sea level.

At waypoint 3 S25° 48.873 E113° 39.421 you will be standing at one end of a Birridas.

1. Describe the distinct colour and texture of the vegetation in the Birridas.
2. As per Q2 of waypoint 2, what is the height above sea level of this Birridas? Keep in mind this use to be sea level.

Email your answers to me via my profile and then log your find. If there are any issues with your answers I will contact you. Logs without emailed answers will be deleted.

Reference
http://www.sharkbay.org/assets/documents/fact%20sheets/birridas%20v2.pdf