Because this sedimentation was so slow it was able to reflect seasonal fluctuations in the calcium carbonate content of the water resulting in thin but very clearly defined light and dark layers or laminae. The light-coloured laminae were deposited during summer and the darker laminae during winter, each pair thus representing an annual increment. By counting these annual increments or 'varves' it is estimated that this particular lake existed for about 220,000 years before it completely silted up to form a marsh or vlei. Fossil evidence of life in the crater lake The accompanying diagram summarizes the types of sediments and the fossils that make up these ancient lake deposits and also illustrates the reconstructed palaeoenvironment. Complete uncrushed shells or carapaces of ostracods are the most abundant fossil, and it appears that they were the first animal to colonize the crater lake.
Ostracods shed their tiny bivalved carapace at least nine times during their growth stages.
Over a period of 6-8 weeks a single population can drop eight times as many shells as there are individuals, thus explaining why certain horizons in the lake deposits are carpeted with discarded carapaces. Ostracods are free-swimming scavengers. They forage around on the lake bottom in search of dead and rotting animal and plant remains. Numerous complete uncrushed fish and frog skeletons are found in the upper parts of the succession. The petrified skeletons are buried or enveloped by successive layers of mud that were able to withstand compaction, thus preserving the fossils in cavities.