Brimham’s Timeline
Brimham’s Timeline
This is a story that started over 500 million years ago involving the movement and collisions of entire continents, of enormous rivers altered by earthquakes and of rocks eroded by ice, rain and wind.
The time scales that cover the story of Brimham Rocks are immense and almost impossible to comprehend, even for geologists, who have split deep time into 22 periods, from the formation of the Earth 4.54 billion years ago to the present day. Brimham’s story starts in the Cambrian Period, about 500 million years ago, with tectonic plate movements but key is the Carboniferous Period, 359 million years ago, when the rivers and deltas that were transporting vast amounts of material from the mountains and depositing the sediments that formed the rocks. And it was during this period 320 million years ago that a braided river system developed, a crucial event in Brimham’s history. The subsequent development of the Pennines during the next 250 million years and the effects of the Ice Ages from 2.6 million years ago have shaped the rocks into the forms we see today.
500 million years ago
Gondwana dominates the planet
The story of Brimham starts about 500 million years ago, during the Cambrian period, as the supercontinent Gondwana began breaking up, torn apart by the slow but inexorable movement of the Earth's tectonic plates. The supercontinent Gondwana started forming between 800 and 650 million years ago, a process completed 530 million years ago when Gondwana covered one-fifth of the Earth’s surface. But tectonic movements continued and by 480 million years ago the small continent Avalonia had broken away from Gondwana and started drifting north to collide with two continental masses, Laurentia and Baltica, forcing the closure of the ancient Iapetus ocean. At this time, during the Ordovician period, the land that was to become Britain lay about 20 degrees south of the equator.
475 million years ago
Mountain building starts
The seven major tectonic plates are not static but move across the globe driven by the heat from the Earth's core. As plates converge and collide, the continental crust, which lies on top of the lithosphere, is forced against the corresponding continental crust of the opposing plate and the entire continental margin deforms and material becomes accreted at the edges. Enormous stresses produce folds, faults, mountain uplift and isostatic adjustments. The huge mass of the accreted terrane can resist subduction and mountain ranges are created. Tectonic movement that causes the collision of oceanic and continental plates results in the heavier oceanic crust being forced under the lighter continental crust, a process called subduction.
475 - 395 million years ago
Mountain building continues
Between 475 million years ago, during the Ordovician period, and 395 million years ago, the Devonian period, there were three main mountain building phases, known as orogeny, as movement of the tectonic plates created such enormous forces that the continents buckled. This forced the land upwards for thousands of feet forming a series of mountain ranges comparable in size to the Himalayas, and which stretched from where Scotland is now to the north of Scandinavia. These are called the Irish-Scottish Caledonides, also known as the Caledonian Mountains, the Scandinavian Caledonides and the Greenland Caledonides.
395 - 321 million years ago
Erosion, rivers & deltas
As soon as the mountains formed, and are exposed to the weather, they were subject to erosion by wind, ice and rain. The rocks were eroded and particles of rock, crystals of quartz and other material were torn from the bedrock and washed into streams and rivers. The rain water and mountain melt-water drained into a series of rivers that flowed in a southerly direction towards an ancient sea, the Paleo-Tethys Ocean.For the next 70 million years, many millions of tons of debris from the mountains, consisting of pebbles, sand, silt and mud, were transported by rivers to the sea as the mountain range was slowly reduced in height. Much of the debris was washed into the sea via a series of large deltas that covered what is now Northern England. ****During this time, the Carboniferous Period, atmospheric oxygen levels were much higher than at present, about 35% compared to 21% today. This allowed insects to grow to enormous sizes, dragonflies with 1 metre wingspans and millipedes 2 metres long. Primitive plants such as ferns, giant horsetails and clubmosses grew to the size of trees, creating great forests that as the plants died formed the coal seams that were so important to Industrial Britain.
321 million years ago
Earthquakes, fault lines & fault relay zones
To the north west of Brimham, deep under the Pennines, is a crustal block of granite, that was created very early on in the Carboniferous period, forming a vitally important element of the Pennine Block and Basin Province. This massive block of immovable rock, called the Askrigg Block, was subject to tectonic forces 321 years ago but being so solid resisted these movements. The rocks around it though, being less rigid, gave way and faults occurred generating a series of earthquakes. The evidence for this can be seen today as the North Craven Fault which runs close to Pateley Bridge on one side of Brimham and towards the Cleveland Basin on the other. The effect of these quakes was massive. As Charles Darwin noted "The Craven Fault, for instance, extends for upwards of 30 miles, and along this line the vertical displacement of strata has varied from 600 to 3000 feet." As the fault developed Brimham was now in the centre of a fault relay zone an area where the ground sloped downwards and the direction of the ancient river flow was altered to follow a generally north/south-west direction.
321 - 319.5 million years ago
Braided river system
With the slope and nature of the ground changing, the effect on the ancient river was profound. No longer a meandering river flowing into a series of deltas, a braided river system developed. This is a dynamic river system with very many channels, sandbars and, critically, easily eroded river and channel banks. The network of river channels are separated by small and often temporary islands called Braid Bars and are characterised by a high slope and sediment load, the banks are unstable and often move. This results in the very rapid formation of bars and channels caused by flood events.
319.5 million years ago
More earthquakes
Not only was the river creating sand banks and carving out river banks but earthquakes were still exerting an influence on the rivers. Part way during the period that the braided river system was active, perhaps 319.5 million years ago, another earthquake shook the ground.The earthquake also had a more profound impact in that the change in ground levels, possibly caused by a further slip in the fault relay zone, forced the rivers to flow in a more westerly direction. This is visible in the rocks at Brimham in two ways. Firstly, the direction of the bedding planes changes in the rocks laid down after the earthquake and secondly there is a visible discontinuity in the layers of rock seen half way up the pillars and blocks that stretches across the site. Yet another earthquake then rocked Brimham generating new earth movements and land slippage pushing the river away and ending the braided river system deposition.It is remarkable that it took just 250,000 years for the sand and pebbles that became the rocks of Brimham to be deposited by the fast flowing, turbulent braided river system. This was just a hundred thousandth part of the entire time between the quartz being formed and the present day.
319 - 298 million years ago
Carboniferous deposits
For the next 20 million years, until the end of the Carboniferous period, Brimham and the rest of the north of England was covered in a succession of deltas, shallow seas and lush swamps. These conditions created layers of sandstone, limestone, shales and coal that would have overlaid the sandstone of Brimham.
298 - 2.6 million years ago
The Pennines
Early in this period, all the continents drifted together to create the supercontinent Pangaea, which by 280 million years ago had formed the largest land mass the Earth has ever known. These earth movements and mountain building events, known as the Hercynian or Variscan orogeny, pushed up the previously flat, level beds of sandstone and limestone into an anticline or dome shape which formed the basis of the Pennines.Since the Triassic, the Pennines experienced a slow uplift followed by extensive erosion, with many thousands of metres of overlying rock being eroded until at Brimham the Millstone Grit constituted the remaining upper layers.
2.6 million - 11,700 years ago
Ice ages
Over the last 2.5 million years the tops of the Pennine gritstone were carved and eroded by the massive ice sheets present during the Quaternary period whilst during the last glacial period, the Devensian, a glacier carved out the Nidd Valley. The ice scoured the overlying rock and soil exposing the blocks of rock where cracks and fissures were created, a process repeated many times as the ice sheets built up and retreated. By the end of this period, Brimham was no longer covered in ice but subject to periglacial conditions of repeated freezing and thawing.
11,700 years ago - today
Erosion
As the ice finally retreated north at the end of the last Ice Age, the rocks that had been exposed by the glaciers were now being eroded by the weather. Rain, wind and the effect of ice in the winter all continued to carve out the shapes seen today. The poor sandy soil and poor drainage prevented the growth of forests or grassland, instead moorland developed. The land was not fit for cultivation and, being on high ground, the rocks at Brimham remained undisturbed until they became a popular tourist attraction during the 18th Century.
