The northern and eastern part of the Wolds and its extension eastwards beneath the coastal marshes is made up of chalk.
This variety of limestone consists of the microscopic remains of animals that made up the plankton of that time. The exact depth of the seawater is difficult to determine, but must have been shallow enough for the seafloor dwelling echinoids, bivalves and sponges to have lived there. Rare ammonites and shark teeth represent life in the near surface waters of the Cretaceous Sea.
At the base of the white chalk a distinctive thin bed of red chalk occurs that extends into Norfolk and East Yorkshire, but does not occur elsewhere in the United Kingdom. Initially the margins of the sea might have been quite close, with land derived materials adding the colouring; alternatively it may be due to the erosion of distinctive red Triassic beds on the floor of the Cretaceous Sea.
The Wolds contain many deep dry valleys cut into a landscape of gently rounded hills, indicating that rivers once flowed through them when the water table was much higher. Settlements mainly occur along the spring lines on the margins of the Wolds where the water table comes to the surface, and the chalk aquifer is a very important source of water here. Prehistoric man was attracted to the flints that occur in parts of the chalk, whilst today the chalk is used for chemical manufacture (the production of lime or mixed with clay to make cement).