These rocks were laid down during the Namurian Epoch (323–313 million years ago) at a time when a great river delta extended across much of northern England. Rivers flowing from high ground to the north created the delta where they flowed into a shallow sea and deposited their load of sand and mud. The constantly changing courses of the rivers and the steadily subsiding delta resulted in over a thousand metres of sediment being deposited. This was eventually converted into rock through burial and compaction.

A delta environment is extremely varied consisting of wide river channels separated by swamps and backwaters. In the deeper, moving water of the river channels, coarse sands formed sand banks or large underwater dunes. Such migratory bedforms are preseved as cross-bedded, gritty sandstones.

Cross-bedded gritstones, Roundhay Park

In the swamps and backwaters between the main channels, mud was the dominant sediment deposited. This has since been compressed to form shales. Their typical black colour is due to the large amount of organic matter they contain, indicating the stagnant nature of the water in which they formed.

Black Shales, Roundhay Park

Evidence of periodic flooding of the delta by the sea is revealed by the presence, usually in the shales, of fossils of the coiled shells of goniatites.