Sediments were accumulated into the basins as clay mixed with plant remains and gastropod fragments. This indicates the presence of plants and animals in the lakes.
Plant remains were then accumulated upon clay layers and became peat and lignite.
In the final stage, clayey sediments were mixed with calcareous components and abundant shell fragements. This is due to huge population of gastropods living in fresh-water lakes. Changes in paleoenvironments and fluctuations of water level resulted in increasing and decreasing numbers of gastropod population as recorded in the fossil beds.
The presence of laterite, laterite soil, river and beach sands overlying mollusc beds indicates that fresh-water lakes became a landmass prior to 200,000 years.