Complex sentence: In some areas, a distinction is made between Upper Greensand and Lower Greensand; these strata tend to occur just beneath the Chalk and can be separated by a layer of Gault.
Split into simpler sentences: In some areas, a distinction is made between Upper Greensand and Lower Greensand. These greensand strata tend to occur just beneath the chalk and can be separated by a layer of Gault.