LITMIR.BIZ

      Limestone pavement on top of Malham Cove (Day 6)

      Even while thick beds of limestone were being laid down, storms were eroding distant mountain ranges. Vast rivers brought mud, sand and gravel down into the sea. These murky deposits reduced the amount of light entering the water, causing delicate coral reefs and other creatures to perish. As more mud and sand was washed into the sea, a vast delta spread across the region.

      At times, shoals of sand and gravel stood above the waterline, and these became colonised by strange, fernlike trees. The level of water in the rivers and sea was in a state of flux. Sometimes the delta was completely flooded, so the plants would be buried under more sand and gravel. The compressed plant material within the beds of sand and mud became thin bands of coal, known as the Coal Measures. This alternating series of sandstones and mudstones, with occasional seams of coal, can be seen best in the Dark Peak and the South Pennines. Remnants of the series can also be studied on the higher summits of the Yorkshire Dales and North Pennines.

      The Carboniferous rocks were laid down in layers, helping to explain what happened next, around 295 million years ago. An extensive mass of molten dolerite was squeezed, under enormous pressure, between the layers of rock – rather like jam between two slices of bread. This rock is always prominent wherever it outcrops, chiefly in the North Pennines and along Hadrian’s Wall, where it is referred to as the Whin Sill.

      Almost 300 million years are ‘missing’ from the Pennine geological record, in which time the range has been broken into enormous blocks by faulting. The Yorkshire Dales and North Pennines display plenty of limestone, as their ‘blocks’ stand higher than the Peak District and South Pennines. Glaciers scoured the entire range during the Ice Age, and many parts are covered with glacial detritus in the form of boulder clay, sand and gravel. More recent climatic changes resulted in the upland soil becoming so waterlogged that thick deposits of peat have formed on most of the higher moorlands.

Pennine scenery

      The underlying geology of the Pennines shapes the scenery along the Pennine Way. The Dark Peak and the South Pennines, whose foundations are sandstones and shales, with gritstone ‘edges’, give rise to acid clay soils, which encourages the formation of thick blanket bog. This bog has been growing for the past 7000 years, but in many places it is decaying, so that the moorlands are riven by peat channels, or ‘groughs’, with high banks of peat between them, known as ‘hags’. Given that the blanket bogs absorb considerable amounts of carbon dioxide, their rapid decay gives cause for concern and efforts are being made to stabilise the remaining bogs and reverse the trend.

      Where limestone dominates, particularly in the Yorkshire Dales but also in parts of the North Pennines, the landscape often looks fresh and green, covered in short, dense, sheep-grazed turf, with bright cliffs and outcrops, or ‘scars’, of limestone poking through. Limestone country is fascinating, mostly because of the way the rock dissolves slowly over the aeons, giving rise to a distinctive landscape known as ‘karst’ topography. Limestone doesn’t just wear down like other rocks but dissolves inside itself, becoming riddled with caves and passages. When these are close to the surface, they may collapse, forming ‘shake holes’.

      In the North Pennines, the existence of the igneous Whin Sill, sandwiched between older beds of rock, forms some of the most striking landscapes in the North Pennines and Northumberland. The mighty Teesdale waterfalls, the striking High Cup and the rugged crest bearing Hadrian’s Wall are all formed by the Whin Sill, which also outcrops along the Northumberland coast.

      Technically, and geologically, the Pennines end just south of Hadrian’s Wall, so the continuation northwards through Northumberland results in another shift in the scenery. While Carboniferous rocks lie underfoot at first, by the time the high Cheviot Hills are reached, the bedrock is either lava or granite. The central granite mass of The Cheviot stands broad-shouldered, with all the other Cheviot Hills huddled around it. The poor acid soil supports thick blanket bog. Many walkers, seeing the Cheviot Hills after spending so long in the Pennines, are surprised at how hilly they are, but this is short-lived, as the Pennine Way ends suddenly with a descent into rolling, pastoral countryside.

The Helm Wind

      Most walkers on the Pennine Way hear about the ‘Helm Wind’ but few understand what it is. The Helm Wind is the only wind in Britain with a name. It only blows from one direction and it gives rise to a peculiar set of conditions. Other winds blow from all points of the compass, but the Helm is very strictly defined, restricted to the East Fellside flank of the North Pennines, and according to local lore, no matter how much it rages, it cannot cross the Eden.