Filling the North Sea
Crucial to the understanding of the present landscape of Scotland is the 60-million-year Tertiary period. With the fall in sea levels and the instability of the land associated with the opening of the Atlantic Ocean came a long period of humid climate. Temperatures varied but were predominantly warmer than those of today. Under these conditions, two formative activities in the evolution of the landscape can be identified: the development of river basins and the weathering of rocks.
Dramatic events were not restricted to the western seaboard. The simple answer to the question "where did all the sediment go which had covered Scotland?" is that it was deposited in the North Sea; an accumulation of up to 3,000m thickness. And why to the North Sea? For two reasons. First, the watershed between the water flowing into the Atlantic and that flowing into the North Sea was well to the west of the country and much of the land drained eastwards; second, tensions in the Earth's crust between Britain and Norway resulted in the formation of a great gash on the floor of the North Sea. It had tremendous capacity because the basin continued to deepen as the forces of the Earth tore it apart. This trench acted as the great sink for the rocks eroded from the surrounding land.
As there are virtually no rocks left on land from this period, we must rely on examination of the material deposited in the North Sea. We can date the deposits but we do not really know how fast the landscape developed. The most plausible view, now of many decades standing, is that major river systems formed on the gently tilted sediments in a classic tree-shaped pattern flowing eastwards. Many of these remain in the present landscape; for example, the Tay, Don and Dee. Gently rolling upland plains were formed, the rivers cut into the ancient rocks and carried their debris into the North Sea.