Q: How were Green and Round Lake formed?

A: The gorge that houses the lake basins was formed by a massive river of water from a melting, retreating glacier during the last ice age (about 15,000 years ago). The lake basins were probably carved out by the force of the river’s waterfalls. After the glacier’s disappearance, the lakes grew to their current shape and size as the pools of water dissolved the surrounding limestone. Today, the lake basins are filled rain and ground water.

Q: What is so geologically “special” about the lakes?

A:  Green and Round Lake are extremely deep, with Round Lake measuring at 180 feet and Green Lake at 195 feet in depth. The waters at the bottom of the lake are high in salinity, and are so dense that they never overturn and intermix with the rest of the lake waters (as typically happens seasonally). Because the lakes do not overturn, they have been designated “meromictic” - making them two of few meromictic lakes to exist in temperate climates. Without seasonal overturn and the stirring up of sediments, the lakes appear extremely reflective and clear. Additionally, sediments are able to collect on the basin, creating a uniquely accurate record of the flora and fauna in the lake and surrounding region. The uniqueness of the lakes caused Round Lake to be designated as a National Natural Landmark in 1975.

Q: What gives the lake their bluish-green color, seen below?

A: The unique color of Green and Round Lake is caused in part by their depths. The only wavelengths that can be dispersed in such depths are on the blue end of spectrum. Additionally, the surface waters of the lakes support strains of bacteria and algae that lend to the water’s blueish-green color, and the lake’s mineral concentration also plays into its appearance. Finally, the colors shine through because the lake waters does intermix. Without the murkiness and muddiness from the sediments that normally kick up during overturn, the lakes possess a still, reflective quality that makes them so aesthetically pleasing. / CC BY 2.0