The rock was formed in a deep sea basin, similar to the trough along present day western side south America, about 300 million years ago
They formed in a deep sea basin called the Clare Basin, that formed during a latter period of mountain building called the Hercynian or Variscan orogeny, when the continent of Eurameric (Laurussia) and Gondwana to collided to form the supercontinent of Pangaea.
The collision resulted in the formation of a subduction zone and the formation a deep marine basin. Episodic submarine landslides, from the continental shelf, would deliver sediments in the deep basin, these avalanches are called turbidity currents. These submarine avalanches were typically set off by earthquakes or powerful storms that destabilised sentiment teetering on the edge of the continental self, sediments of the accretionary prism. The turbidites now form the lighter sandstones and fine conglomerates of the sea stack.
In between these avalanches of marine sediment, are dark shales, formed over hundreds or thousands of years as the deep sea calmly and slowly accumulated fine grained organic rich sediments that rained down from the surface.
As a whole, the turbidites and shales are known as a flysch sequence.