The Wolds comprise a series of low hills and steep valleys underlain by calcareous chalk, green limestone and sandstone rock, laid down in the Cretaceous period under a shallow warm sea. The characteristic open valleys of the Wolds were created during the last ice age through the action of glaciation and meltwater. Geographically, the Lincolnshire Wolds are a continuation of the Yorkshire Wolds which run through the East Riding of Yorkshire, the Wolds as a whole having been bisected by the erosive power of the waters of the Humber.