While you will be conversant with maps and route descriptions, that familiarity may not extend to an understanding of rocks and landforms. They present special challenges because the enormity of geological time is difficult to appreciate, as is the scale and frequency of change that has taken place on the Earth’s surface over time. Our assumption is that you may have no particular geological knowledge, but are not averse to being introduced to it. At various points on the walk the narrative draws your attention to features of interest. In attempting to describe and explain those features we have tried to avoid too much jargon, but inevitably some terms and concepts may be unfamiliar to you. To help, we have identified the technical terms in bold and will link them to an online glossary in the near future. In the meantime, we have provided a list of explanatory books under ‘Further reading’ that may further clarify the picture.
The concept of geological time is both interesting and important. It is usually expressed in two different ways. The relative age of a rock refers to the physical subdivision of all the rock sequences found on Earth, based on their position and the fossils they contain. These subdivisions are given names that you may be familiar with, such as ‘Cambrian’ and ‘Jurassic’, and they are known as geological time periods. They are usually depicted in a column where their relative position indicates that younger rocks are stacked on top of older rocks.
The second expression of geological time is called an absolute age and it is a numerical value, such as 300 million years. These numbers are obtained by radiometric dating of suitable rock types in order to work out how old they are. In practice, absolute ages are used to calibrate the relative age of a rock, resulting in the geological time scale shown here. You will see that it is simple enough to determine that the Jurassic period of Earth’s history occurred after the Triassic and before the Cretaceous, and that it spanned a time interval from about 200-150 million years ago (Ma).
What’s in a name
It is perhaps surprising that, despite the enormous antiquity of the rocks in the Peak District, only one geological time period is represented to any great extent. That is the Carboniferous. Older rocks are deeply buried and hidden from view, while younger rocks were once present across the area but have subsequently been almost completely removed by natural processes. The Carboniferous refers to a span of 60 million years between about 360-300 Ma. During that time a wide variety of rocks were deposited in the Peak District, the detail of which is bewildering for all but a specialist.
For that reason, in this guide — which is for walkers rather than geologists — we refer to a suite of names based on the observation that rock sequences dominated by limestone belong to the Early Carboniferous, while the overlying rocks that are characterised by sandstone and mudstone are assigned to the Late Carboniferous. Within that much simplified framework, many rock sequences are distinctive and can be recognised locally, such that their distribution can be shown on a geological map. They are generally named after places where they are best seen and the name often incorporates a description of the rock type itself. The result is a very useful, often self-explanatory set of local rock formation names and we use these throughout the guide; Eyam Limestone and Chatsworth Grit are good examples.
The chart shows the names of important rock formations in the Peak District, how they stack on top of each other, and therefore their relative ages. However, it’s important to emphasise that the complete Carboniferous rock succession cannot be seen at any one place, but rather is established through detective work by geologists. As you proceed around the GeoWalk you’ll see examples of each named rock formation and learn something more about them.
Looking back in time
The limestone of the Peak District formed during the Early Carboniferous when Britain lay close to the equator. Most of the Pennine Basin was covered by a warm shallow tropical sea which teemed with life such as corals, seashells and microscopic shelly organisms. As those creatures died, their carbonate skeletons accumulated on the sea floor and were broken up by wave action before being buried and converted into the hard, pale-coloured limestone that characterises the White Peak. During this time volcanoes were also intermittently active in this area, depositing layers of ash and lava.
Millions of years later, the limestone was gradually overlain by mud and sand that was dumped by huge rivers that drained upland areas to the north and east of Britain. The rivers built up large deltas as they flowed into the sea and some areas of the coastal plain supported lush vegetation. In time these dark-coloured sediments and plant-rich layers gave rise to mudstone, sandstone and coal that now characterise the Dark Peak. The various sandstones that dominate this Late Carboniferous succession are often collectively referred to as ‘Millstone Grit’ because they have been used in the past for industrial purposes.
Around 300 Ma, the whole rock sequence was folded upwards into a dome-like structure by powerful earth movements. Then followed long periods during which the rocks exposed at the Earth’s surface were either worn away by wind and rain, or submerged beneath the sea and covered by layers of younger sediment. In total, several kilometres of rock have been eroded from the Peak District, particularly during the last 40 million years, to reveal the central core of Carboniferous limestone and remnants of the overlying mudstone, sandstone and coal around the flanks.
In contrast to the rocks themselves, the landforms of the Peak District are relatively young features. They started to form as the cover rocks were progressively removed, with the underlying hard, strong rocks resisting erosion and remaining upstanding as hills. Softer rocks were worn down and washed away to create the intervening valleys. The finishing touches to the landforms have been created during the last one million years, during periods of global climate deterioration known as Ice Ages that were characterised by plummeting temperatures, expansion of the polar ice-caps and a fall in sea-level. There were also warmer spells during the ice ages and the alternations between freezing and temperate conditions were particularly instrumental in the development of river systems, dry valleys, caves, landslides, screes and tors.
Mam Tor © Julie Harrald