Halkyn Mountain is described as a limestone plateau, although its geology is far more complex than that with layers of chert, millstone grit, clay and sand and gravel.
A Tropical Sea
The limestone was formed 340 million years ago when a shallow tropical sea covered the area. The shells of sea creatures built up on the seabed over thousands of years.
The climate slowly changed and other layers formed above, over millions of years, gradually compressing the shells to form limestone. Many fossilised marine plants and animals are found in the limestone.