formation1

The story of Dartmoor’s formation is one stretching across at least 400 million years. It is a story of oceans, plate tectonics, volcanoes, mountains, deserts, tropical rain forest, and ice ages.

The Devonian 408 to 363 Ma

We start our journey in the Lower Devonian period 408-386 million years ago. Devon was then part of the continent of Laurasia, (Old Red Sandstone or Caledonian) forming part of the coastline. To the north a semi-arid mountainous land fed rivers into the southern sea and today’s rocks known as the Dartmouth slates were laid down in fluvial conditions.
In basin areas lakes developed where fine material settled out, these muds became mudstone which later became slate.

The sea encroached on the land and Devon became part of the Rheic sea, an elongated marine trough, in which an ocean was to develop. The Meadfoot beds record this as shale laid down on the sea bed.

By the Middle Devonian, in shallow water and around volcanic islands, limestone was forming. Some of these limestones contain reef forming fossils known as stromatoporoids. As ocean crust begins to develop to the south the upwelling mid-ocean ridge produces basalt. A slice of this ocean floor would later become the Lizard Complex in Cornwall.

The Upper Devonian, 377-363 million years ago saw a continuation of similar circumstances. However, to the south the large continent of Gondwanaland was moving steadily north to eventually meet with Laurasia.
Normannia, part of this southern continental mass was being thrust up into a leading edge of mountains, with huge rafts of rock being thrust one over another. Ahead of this was an ever-deepening marine trough into which the Late Devonian and Early Carboniferous sediments would be deposited.

Too deep for limestones to form the sediments would become chert, sandstone and siltstone, with jumbled up sediments known as turbidites formed on the continental slopes by under sea avalanches. The scene was set for the next great period of Earth history, the Carboniferous.