When the crust of the Earth eventually cooled, there began the most tremendous erosion of its surface rocks.
Under the non-stop, pelting rain, the softer rocks broke up and were swept away. The first valleys were formed, along which the sheets of water rushed towards the largest hollows in the surface of the Earth. Here it collected as it fell from the sky or ran down off the mountains. The hollows filled up and became wider and wider water basins from which islands and erupting volcanoes appeared. And so the first seas and the primeval oceans were formed.
The greatest development of the oceans took place at the start of the Palaeozoic Era, which was the time of the great rains. Whole continents were submerged by the water, which spread over much of the Earth’s surface.
The rule of the seas lasted for another hundred million years, through the Cambrian and into the Ordovician Period. A new series of major foldings of the Earth’s crust then began, forcing up great mountain chains. The formation of the mountains of Scandinavia, Scotland and Ireland dates back to this period.