Читать книгу Short walks in the Lake District - Collins Maps - Страница 8

The history of the Lake District over the last 530 million years can be seen in the landscape at the present time.

The oldest rocks are the Skiddaw Slates. These were formed from sediments of gravel, grit and mud laid down in a shallow sea and later subjected to great pressure. Their age is usually put into the Ordovician period some 530 million years ago. Most of this rock series now resembles shale rather than slate. It is minutely jointed and with the action of weather and frost has broken down into small flakes.

After the deposition of the Skiddaw Slates came several millions of years of volcanic action. Lava burst out from below the surface and flowed over the landscape. Explosions threw out hot ‘bombs’ of rock. Ash settled to huge depths. All this mass of varied material settled to a depth of up to 2 miles (3km) and formed what are known as the Borrowdale Volcanic series of rocks.

After that the whole area was covered by a shallow sea, and erosion material, at first calcareous, then huge amounts of grit and mud settled in layers to a depth of 2½ miles (4km). This was in the Silurian period between 440 and 410 million years ago.

It can be imagined that the then lakeless Lake District consisted of three very thick layers of material each on top of the other. During the Devonian period the earth’s crust was subject to extreme movement. The area was thrust upwards into a dome. Each of the rock types reacted differently according to the position of the centre of the thrust. Once the upper layers were fractured, for instance, the more malleable Skiddaw Slate beneath was pushed through to a great height, then hot sandstorms and heavy rains wore down the upper layers. Much of Skiddaw Slates’ topmost parts, and a substantial area of the Silurian rocks were swept away. In the central parts the much harder Borrowdale Volcanics were left exposed. Therefore the Skiddaw Slates were left uppermost in the north, the Volcanics in the centre, and the Silurian to the south.

From 345 to 280 million years ago there followed the Carboniferous period. The area was again covered by a sea rich in life. The central part of the district probably remained as an island. The deposits in this sea formed the carboniferous limestone. Subsequently much of this was swept away leaving a rim around the Lake District. The Permo Trias period followed; the area became hot and arid. The desert sands of this time were later solidified into the New Red Sandstone, which again was mainly swept away from the central dome.

The Tertiary period, 65 million years ago brought new upheavals to the planet. This ‘Alpine’ movement produced the Alps, the Himalayas and the Andes. Again the Lake District’s dome was raised high. Fracture lines appeared, in general radiating from the centre, but varying in direction according to the reaction of the material. These formed the basis of the valley patterns we know today.

The next great catastrophe was nearer our own time. About one a half million years ago the climate changed dramatically and the whole of the northern hemisphere was covered in ice. The subsequent movement and melting of the ice hollowed out the valleys and lake beds, and swept away vegetation. The heavy rainstorms later moved any remaining loose material. Thus the dales were sculpted and the lakes were formed.