The Wolds are formed largely from a series of pure marine
limestones formed during the
Cretaceous period, known collectively as the
Chalk Group. The chalk overlies a series of other
sedimentary strata of late Jurassic/early Cretaceous age. The strata dip gently to the east and form a scarp which runs southeast from the south bank of the
Humber just west of the small town of
Barton upon Humber via Caistor before it loses its identity just south of the small town of
Spilsby. To the north of the
Humber Gap, the same formations continue as the
Yorkshire Wolds. During the
last ice age (Devensian), ice encroached upon the Wolds from the northeast and entered the Humber gap from the east but did not cover the Wolds, hence there is no Devensian age glacial
till on these hills. However, an earlier ice age left extensive spreads of till across the central and southern areas. Sections of the main scarp, notably at Saxby All Saints and between Nettleton and Walesby have been subject to
landslip. ==Geography==