Peak District - Geology

Geology

The Peak District is formed almost exclusively from sedimentary rocks dating from the Carboniferous period. They comprise the Carboniferous Limestone, the overlying Gritstone and finally the Coal Measures, though the latter occur only on the extreme margins of the area. In addition there are infrequent outcrops of igneous rocks including lavas, tuffs and volcanic vent agglomerates.'

The general geological structure of the Peak District is that of a broad dome (see image below), whose western margins have been most intensely faulted and folded. Uplift and erosion have effectively sliced the top off the dome to reveal a concentric outcrop pattern with Coal Measures rocks on the eastern and western margins, Carboniferous Limestone at the core and with rocks of Millstone Grit outcropping between these two. The southern edge of the dome is overlain by sandstones of Triassic age though these barely impinge upon the National Park.

The central and southern section of the Peak District, where the Carboniferous Limestone is found at or near the surface, is known as the White Peak in contrast to the Dark Peak, which is characterised by Millstone Grit outcrops and broad swathes of moorland.

Earth movements both during and after the Carboniferous period resulted in the up-doming of the area and, particularly in the west, the folding of the rock strata along north–south axes. The region was raised in a north–south line which resulted in this dome-like shape and the shale and sandstone were worn away until limestone was exposed. At the end of this period, the Earth's crust sank here which led to the area being covered by sea, depositing a variety of new rocks.

Some time after its deposition, mineral veins were formed in the limestone. These veins and rakes have been mined for lead since Roman times.

The Peak District was overrun by ice during at least one of the ice ages of the last 2 million years (probably the Anglian glaciation of around 450,000 years ago) as evidenced by the patches of glacial till or boulder clay that can be found across the area. It was not, however, covered by ice during the Last glacial period, which peaked around 20,000–22,000 years ago. However a mix of Irish Sea and Lake District ice did butt up against its western margins. Glacial meltwaters eroded a complex of sinuous channels along this margin of the Peak District during this period. Glacial meltwaters also contributed to the formation and development of many of the caves in the limestone area. Wild animal herds roamed the area, and their remains have been found in several of the local caves.

The different types of rock that lie beneath the soil strongly influence the landscape; they determine the type of vegetation that will grow, and ultimately the type of animal that will inhabit the area. Limestone has fissures and is soluble in water, therefore rivers have been able to carve deep, narrow valleys. These rivers then often find a route underground, creating cave systems. Millstone Grit on the other hand is insoluble but porous, so it absorbs water which often seeps through the grits, until it meets the less porous shales beneath, creating springs when it reaches the surface again. The shales are friable and easily attacked by frost, so they form areas that are vulnerable to landslides, as on Mam Tor.