|
|
|
 |
| |
| North Pennines |
| |
| Habitat: Earth heritage (of national significance) |
| |
The rocks, fossils, minerals and landforms of geological interest, together with the natural geomorphological processes that continue to shape the landscape.
|
|
| |
The underlying geology of the Natural Area is dominated by Carboniferous Limestone, thenature of which is complex and which gives rise to an open and rugged landscape. TheCarboniferous rocks are divided by geologists into the following sequence (in ascendingorder): Carboniferous limestone (Dinantian, 350-333 Ma), Millstone Grit and Shale(Namurian, 333-318 Ma), and Coal Measures (Westphalian, 318-303 Ma). The boundarybetween the limestones and grits is known as the Yoredale Succession, and this is made up ofa rhythmic sequence of limestones, shales, sandstones, grits and coal. This sequence indicatesthe changes in the seas during the Carboniferous period from clear water oceans (limestones),through increasingly muddy seas (shales) to swampy estuaries and deltas (current-beddedsandstones and grits). Glacial deposition has also been extensive and alluvium has developedon valley floors, giving rise to a wide range of soil parent materials.
The plateau which comprises the Natural Area arose when the so-called Alston Block wasuplifted during seabed movements in the late Carboniferous, with resulting faults to the west(the Eden Scarp) and the north (the Tyne Gap). Also during the Carboniferous, volcanicactivity intruded great sheets of dolerite into the limestones in the form of sills. These layers(the Whin Sill) now form highly resistant layers in the limestone and survive as vertical‘steps' in the landscape. The various seabed movements and volcanic activity has had acontrolling influence on later patterns of sedimentation and mineralisation which, in turn, hascreated extensive veins of lead and zinc within fissures in the limestone.
The impervious rocks of the Natural Area support numerous surface rivers and streams whichhave, over many centuries, given rise to extensive underground caves where they drain into thelimestones beneath. Some of these have been designated as geological Sites of Special ScientificInterest, notably at Fairy Hole Caves and the maze cave of Knock Fell Caverns (at Moor Houseand Cross Fell SSSI).
On higher ground the rock is covered by a thick layer of peat which, in turn, rests upon glacialclay. In the higher, wetter, western areas this appears as blanket bog, whilst in the lower and drier east it supports heather moorland or acidic grassland. Where the limestone breaks through, in extensive scars or along deeply incised river valleys, richer soils have developed, supporting a varied lime-loving vegetation; in the far west of the Natural Area, above Brough, the limestone outcrops as pavement.
A particularly significant geological feature of Upper Teesdale, towards the southern part of theNatural Area, is an intrusion of the igneous Whin sill into the limestone, the result of which wasthe baking of the limestone into a type of marble. This has itself become weathered into the so-called " sugar limestone" soil which supports the unique Teesdale flora.
|
| |
|
