A layer of silt lying on top of a layer of sand represents 1 year’s deposition in the lake and is known as a varve. Lake deposits can show a distinct layering (stratified).
The coarser layer (lighter colour) is the spring/summer deposition when meltwater is at its peak and is carrying maximum load.
The thinner layer (dark colour) of finer silt settles during autumn and winter as stream discharge decreases and the very fine sediment settles to the bottom.
They’re a good indicator of the age of lake sediments and of past climates as the thickness indicates warmer and colder periods.