Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the ‘Lower Carboniferous Period’), when sea temperatures were much higher and the coastline at a greater distance away.
As a fun little addition, look about a third of the way up, where the clean horizontal lines of the outcrop are broken by a small arc, thick in the middle and pinching out towards the sides, concave upwards and with a darker/black color.
That’s a fluvial deposit, probably a small creek or river- the arc is a cross section, so the lowest point is the deepest part of the river. It’s darker because it was rich in organic matter, and after the plant matter was buried here the constituent carbon remained for millions of years- possibly as a small lignite or coal deposit. (‘Carboniferous’ period, right? It’s a common sight in this age of rock)
This is a 350,000,000 year old riverbed. Fun to imagine standing by that ancient creek, ferns and primitive moss growing thick in the shallows, insects and the odd critter enjoying themselves on the banks, and knowing that something of this moment would remain even three hundred and fifty millions years in the future, embedded in a monolith rising above the sea.
