The beautiful blue-green colour of Emerald Lake is created by sunlight reflecting off a white layer of “marl” on the lake bed. Marl is a white calcium carbonate (CaCO3) clay that forms in the water and then settles, often unevenly, onto the lake bottom. It forms when enough carbonate (CO3), from dissolving limestone), reacts with calcium (Ca) in the water.
The limestone hills around you were created 150 – 200 million years ago by reef-building corals growing in a warm sea. Corals and lime-secreting algae cemented themselves together in 100 metres of water where there was sunshine and clear water. As the water gradually rose the coral reef built upon itself, growing toward the light.
This valley was covered by a glacier during the last ice age and shallow lakes were formed when the glaciers retreated 14,000 years ago. Retreating ice deposited limestone gravel, eroded from the surrounding hills, on the valley floor. The carbonate rich gravel affected the ground water and led eventually to the formation of marl in the lake.
Diatoms are an algae commonly found in shallow lakes like this one. They form a unique external structure composed of calcium carbonate from the water and when they die, their shells contribute to the layer of marl. If you could look closely at the marl layer you would see tiny land snail shells and many diatom cases such as the one illustrated here.