Hot Springs and Geysers @ Rhynie & Yellowstone National Park, USA

Today, the small village of Rhynie in Aberdeenshire is surrounded by gently rolling hills and fertile farmland. Behind Rhynie, sits the ‘Tap ‘o Noth’, a distinctive hill, identifiable for miles around due to its flat top, the site of a vitrified hillfort - because of its shape and the blocks of fused rock, the hill is sometimes mistaken for an extinct volcano. But beneath the unassuming farmland between the hill and the village, lies one of the world’s best known fossil sites. This site dates back to around 410 million years ago, during the Early Devonian, a time also known as the ‘Old Red Sandstone’.

View of the Tap o’ Noth near Rhynie.
Photo © Diane Mitchell, National Museums of Scotland.

View of the village of Rhynie looking east from the side of Tap o’ Noth.
The Rhynie Chert lies beneath the fields in the foreground.
Photo © Dr Lyall Anderson, National Museums of Scotland.

In Scotland today, we have many Devonian rocks, both sedimentary and igneous in origin. But the rocks at Rhynie are unique and internationally important, thanks to a sequence of beds that don’t even outcrop above ground. Over 33 thin bands of rock called the Rhynie Cherts comprise the oldest known, preserved terrestrial hot spring system in the world. These hot springs would have been similar in appearance and size to those found today at Yellowstone National Park in USA, and are the reason for this twinning.

This is a cut and polished piece of Rhynie Chert.
At the top, we can see layers –
formed by the gradual growth of sinter over time.
Photo © The Trustees of the National Museums of Scotland.

However, it is the fossils within the chert that makes the site so important. When the hot springs were active, the water was rich in dissolved silica *. When the nearby plants and animals got wet, they became coated in the silica. When the water cooled, the silica crystallised out, and the plants and animals were then protected from air and further bacterial decomposition. Because of this, they did not decompose – unlike most fossils – and so became preserved in incredible 3D detail. The Rhynie Chert has produced the oldest known ‘insect’ fossils, some of which were the earliest colonisers of land. Unsurprisingly, research on the Rhynie Chert has been ongoing for a number of years and has given scientists a fascinating and crucial insight into this early ecosystem.

Read on to find out more about Scotland’s own ‘Yellowstone’.

* Silica is the chemical element which, along with oxygen, forms the mineral called quartz. Both chert and sinter are forms of quartz.