Some of the youngest rocks in the National Park make up the Tabular Hills, which form the southern edge of the moors. These limestones are around 150 million years old and were formed at a time when warm shallow seas covered North Yorkshire. Large coral reefs formed much of the sea bed while ammonites and marine reptiles inhabited the open water.
The sloping flat tops of the Tabular Hills are due to the underlying beds of limestone dipping down towards the south. These gentle slopes are occasionally broken by deep gorges, such as Newtondale and Forge Valley. These are glacial run-off channels formed when huge amounts of melt water poured down the lowest points in the landscape during the last Ice Age.
One of the strangest features in the Tabular Hills is the Hole of Horcum. This scallop-shaped valley has been formed by the action of a line of springs along the boundary between two rock layers.
Dividing the Tabular Hills from the central moorland is a distinctive escarpment. This ridge marks the boundary between Upper and Middle Jurassic – it affords great views at places like Saltergate and runs along the width of the moors from Bilsdale to the coast. This escarpment is a result of the action of streams and weathering on the uplifted rocks.