During the Carboniferous, the UK was well within the tropics, and formed part of the southern boarder of a land mass which included North America, Greenland and Northern Europe (the 'Euramerican Block').
The Midland Valley of Scotland (including Edinburgh and Glasgow) and the north of England formed relatively low-lying ground, with vast, meandering river systems draining the northern uplands. South of Northumbria the UK was generally under a shallow sea, with the exception of an occational tropical island (e.g. one around Derby). This type of lowland at the edge of a continent is called a 'coastal alluvial plain'. The river systems were surrounded by huge swampy forests of ancient tree-like plants. During times of heavy rainfall the rivers would burst their banks and create temporary lakes. There were also perminant lakes, such as Lake Cadell around Edinburgh. Over the centuries Lake Cadell fluctuated from being a series of isolated pools to being a single coherant mass of fresh water covering several thousand square kilometres. Because it was so low-lying, the plain was also occasionally flooded by the sea. Finally, changes in sea level over thouands of years gradually shifted the coastline back and forth.

The plants in these forests (which also contained a rich fauna of invertebrates and some of the earliest tetrapods) did not completly decay after death, but gave rise to huge deposits of coal, which have been extensively mined throughout Europe and America. The regions under the shallow sea laid down fine silts which were to become the Carboniferous limestones of the UK.

Three species of rhizodont lived in the rivers and lakes of the UK (Rhizodus hibberti - 7 metres long; Strepsodus sauroides - 5 metres long; Screbinodus ornatus - 2-3 metres long). Their prey probably included larges sharks, lungfish and other lobe-finned fishes, and even tetrapods (all tetrapods at this time still had to lay their eggs in water; there were also some secondarily aquatic species).