Looking around the upper Tweed valley with its hills, river and burns you may wonder how this landscape came to be in its present form. The answer lies in a combination of the underlying rocks and the many millions of years that have passed since they were deposited.
The rocks that we stand on were deposited at the bottom of the Iapetus Ocean during the Ordovician and Silurian periods, between 490 and 415 million years ago. At this time the ocean lay between two contents, Avalonia and Laurentia, with England and Europe on one side and northern Scotland together with North American on the other. The continents were not in their present location but somewhere near the equator. Through the process of continental drift, which drives the continents around the earth’s surface on convection currents in the molten mantle, the two continent moved together thrusting up the sediments from the sea floor and forming a mountain range in the same way as the Alps and Himalayas. The rocks of these mountains contained abundant marine fossils which have allowed the ages of the rocks to be identified and for the complex of strata to be unravelled. The most important group of fossils are the long extinct graptolites which were colonial animals which floated in the ocean feeding on plankton. These fossils have two major advantages when used to date the rocks that they are found in, firstly being planktonic, they were present throughout the ocean, and secondly they had a rapid rate of evolution which resulted in individual species having short periods during which they existed.
From the time of mountain formation to the present day, the forces of water, wind and ice have eroded the land to its present level. As a result there are no rocks in Tweeddale representing the period of over 400 million years between the Silurian the post-glacial deposits which are mainly less than ten thousand years old. Faults and fractures in the Earth’s crust have provided week points which have been progressively eroded over great periods of time by rivers and later by glaciers.
The result has been to define the path of the River Tween in a valley described by fault lines, eroded by successive rivers and the ice of the Pleistocene glaciation.
The upper Tweed valley shows a variety of well developed glacial features from the classic ‘U’ shaped glacial valley of Talla to the glacial gravel terraces which line the floor of the present.
The product of nearly 500 million years of geological processes now leaves us with the landscape of today.