The Devonian Period

The Devonian Period followed the closure of the Iapetus Ocean when early versions of North America and Europe collided, forming the continent of Euramerica and the huge Caledonian mountain chain. The remains of these mountains can be seen as far afield as North America, Ireland, Scotland and Scandinavia today.

The period is named after Devon, in southwestern England, where marine sedimentary rocks of this period were first studied. Elsewhere in Britain the Devonian is represented by terrestrial sediments comprising mainly distinctive red and brown sandstones collectively known as ‘Old Red Sandstone’.

The Devonian extends from 416 million years ago to 359 million years ago, although Shetland’s Devonian rocks were formed from 394-384 million years ago. At this time, the rocks of northern Britain lay just south of the Equator and far inland, with mountains to the northwest and the open sea to the south east. Between the mountains and the sea was a vast desert plain.

A large shallow freshwater lake - Lake Orcadie - formed within a broad basin and was fed by rivers flowing from the mountains. Shetland lay at the edge of the Orcadian basin in the eroding mountain foothills to the north of this lake. It would have been relatively low-lying compared to other parts of Scotland.

Boulders, pebbles, sand and mud were washed down from the mountains by fast flowing torrents. At the base of the mountains the gradient eased so the water no longer had the energy to move larger stones and these were dumped. The finer material was carried out into the plain by braided rivers and eventually deposited on floodplains and in other lakes. These sediments formed the conglomerates, sandstones and mudstones that can be seen in the south and west of the islands today.

The climate varied from warm and humid to dry and arid in cycles of several thousand years driven by the Milankovitch cycles of Earth’s orbit round the Sun. It was a volcanically active time throughout Scotland and volcanic rocks from this period can be found near Oban, in Fife, and east of Jedburgh in the Scottish Borders as well as on Shetland. The exposed remains of magma chambers that once fed the volcanoes can also be seen in Shetland and many parts of Scotland.

The Devonian period saw great changes in both terrestrial and aquatic life, and the evolution of a remarkable diversity of fish – including bizarre-looking bone-plated creatures with powerful jaws lined with bladelike plates that acted as teeth. Fish like these inhabited the shallow margins of lakes within the Orcadian basin, having probably colonised at times of high water levels when rivers connected the lakes to the sea.

Fossilized fish remains can be found in the Devonian lake sediments of Shetland, Orkney and Caithness. Often they are preserved almost intact, having sunk into the oxygen-starved depths of the lake where they wouldn’t be disturbed by scavengers before they were buried.

Plants began spreading beyond the wetlands, with new types developing that could survive on the margins of rivers and lakes. Toward the end of the Devonian the first “forests” arose as stemmed plants evolved strong, woody structures capable of supporting raised branches and leaves. Many plant fossils have been found on the island of Bressay and elsewhere in Shetland.

The Devonian period ended with a mass extinction. Life on land was not badly affected but up to 70% of marine species were wiped out, possibly because of the rapid spread of newly-evolved terrestrial plants which removed CO2 from the atmosphere, causing global cooling.