500 million years ago - Mountain Building
The story of Brimham Rocks starts 500 million years ago as the supercontinent Gondwana began breaking up, torn apart by the movement of the earth's tectonic plates. About 480 million years ago in the part of the globe that was about 20 degrees south of the equator the small continent of Avalonia started to drift northward and collided with two continental land masses, Laurentia and Baltica. Over the next 100 million years this collision created such enormous forces that the land buckled and a series of mountain ranges the size of the Himalayas were formed.
395 - 321 million years ago - Erosion, Rivers and Deltas
As soon as the mountains are formed, they are subjected to the effects of the weather. Rain, wind, ice and snow erode the summits and crags and the mountain streams flow into the rivers carrying the eroded rock debris. The rivers flowed in a southerly direction towards the Paleo-Tethys Ocean and emptied via a series of massive deltas that covered much of what is now northern England. The sand, silt and pebbles carved by these rivers consisted primarily of quartz, a very hard mineral resistant to erosion, which formed layer after layer of sediments. As these sediments dried out and were subject to immense pressure from the weight of overlying layers they underwent a series of changes to form the sandstone rocks called Millstone Grit that is found throughout the Pennine region and at Brimham.
During this time, the Carboniferous Period, Britain lay about 20 degrees south of the equator. The atmospheric level of oxygen, at 35% was much higher than today's 21% which enabled insects to grow to enormous sizes. Dragonflies with a wingspan of 1 metre cruised overhead and millipedes up to 2 metres long crawled through the forest undergrowth.
321 million years ago - Earthquakes, Earth Movements and Braided Rivers
The most significant events in the story of Brimham occurred between 321 and 319 million years ago. The first of these was a series of earthquakes the results of which are still visible today as the North Craven fault. The quake fractured the rocks some of which slipped downwards by up to 1000 metres along the fault line. This earth movement forced the ancient river to change the direction of flow and, most importantly, to change the type of flow. The river no longer meandered into a delta but became an extremely dynamic braided river where the river banks were rapidly eroded, river bars and sand bars were formed and quickly washed downstream and the pebbles were deposited by numerous flood events.
319 million years ago - Carboniferous Deposits and the Pennines
After just 250,000 years another earthquake shook the land and the ground levels again shifted, though this time the effect was to move the dynamic river away from the area. During the next 20 million years or so Millstone Grit continued to be deposited covering Brimham and most of northern England in massive layers of rock. Further tectonic shifts created an upthrust that raised the Pennines and, in particular, the rocks overlying Brimham to the point where the effects of millions of years of erosion wore away these layers.
2.6 million years ago - The Ice Ages
The final part of the story revolves around the effects of the numerous ice ages of the last 2.6 million years, which eroded the overlying layers of rock sediments leaving the Brimham rock layers exposed after the last ice age which ended just 11,700 years ago.
With the rocks no longer covered, rain, ice and wind directly erode the blocks creating the remarkable sculptured shapes we see today.
The cliffs that form the western edge of the site, at spot called Lover's Leap, were carved out by a glacier that created the valley in which the River Nidd flows.
The Dancing Bear shows very clearly the layers of rock that were deposited by the braided river. The effect of wind erosion has accentuated each layer, called a bedding plane, as not all layers erode at the same rate.
As the Millstone Grit rocks are eroded, the hard quartz grains fall to the ground only to be picked up by the wind and blasted against the rock surface. These sand grains are heavy and tend to have the greatest effect closer to the ground. Over time, the lower part of the rock will be eroded faster than the top and these fantastical shapes are the result.
As you walk round Brimham, the grains of sand crunching under your feet are very much part of the 500 million year story from the breakup of an ancient continent to mountain building and the effects of wind, rain and ice.
Photos of Brimham and text by Stephen Lewis.