Some of the oldest rocks in the Midland Valley region date from the Ordovician period, around 470 million years ago. At that time northern Scotland lay at the southern edge of a continent known by geologists as Laurentia with the area that was to become the Midland Valley, forming a line of island volcanoes in the adjoining and gradually closing Iapetus Ocean. The Iapetus Ocean closed during Silurian times, in a continental collision known as the Caledonian Orogeny. This joined the crustal foundations of both Scotland and England.
Erosion of the mountains to the north and south, produced sand, silt and mud that was carried in to the Midland Valley area, covering the remains of the volcanic island chain. By Devonian and early Carboniferous times, Scotland lay just south of the equator. The climate was hot and dry, with seasonal rains. Rivers laid down ‘The Old Red Sandstone’ and volcanic activity gave rise to extensive lava fields.
During the Carboniferous, around 320 million years ago, Scotland lay on the equator with a tropical climate. Corals grew on reefs in warm shallow seas and swampy forests that flourished in a ‘Greenhouse’ atmosphere, gave rise to coal. Volcanoes such as Arthur’s Seat in Edinburgh dotted the landscape.
When Scotland lay in the desert belt north of the equator, during Permian-Triassic times around 250 million years ago, sand dunes and flash flood deposits were laid down. These can be seen in Ayrshire and Arran.
The remains of a 60 million year old volcanic complex on Arran, represents a time when there was intense volcanic activity along the western margin of Scotland. This occurred when north-west Europe was being split from North America, with the formation of the North Atlantic
This area contains a wide variety of lowland glacial landforms and deposits, coastal landforms and deposits and active river landforms.
The effects of glacial erosion in this area are more subtle than in the Highlands. Pre-existing valleys were over-deepened but later infilled with drift, and the hills moulded and streamlined by the passage of the ice. Striking examples of such moulding are represented in the crag and tail forms and streamlined bedrock of Midlothian and East Lothian. The northern mountains of Arran display many classic landforms of mountain glacial erosion.
The lower ground is mostly covered by a cover of drift deposits, comprising of till and sand and gravel, from the last and earlier ice sheets. These deposits have a range of surface forms, including, eskers, kames and outwash terraces. Notable examples include the drumlin field of the Glasgow area and the nationally important Carstairs Kames.
The Loch Lomond area provides evidence for the final episode of glaciation at the end of the ice age. This is known as the Loch Lomond Readvance, a glacial event which occurred between about 12,600 and 11,500 years ago.
A distinctive aspect of the coastal scenery in the west is the pronounced shore platform and its associated cliff line, part of the Main Rock Platform. The western Forth Valley has one of the most important records of relative sea-level change in Scotland. Here a sequence of buried beaches and carse deposits provide a record of changes during the last 12,000 years. The coastal lowlands of Angus, Fife and East Lothian provide widespread evidence of sea level changes, in the form of raised shorelines and the extensive development of carselands.