Discover Scotland and Explore the Geodiversity of the Achnasheen Terraces Near Wester Ross
- Wayne Munday
- Jun 6
- 3 min read
Sip back and discover Scotland and explore the Achnasheen Terraces a designated Site of Special Scientific Interest (SSSI) located just south of the village of Achnasheen in the Northwest Highlands of Scotland. Covering approximately 2.5 km² this site is one of Scotland’s most striking and well-preserved examples of a fluvioglacial landscape showcasing a series of geologically significant outwash and delta terraces with dramatic ice-contact slopes. This was once a dynamic environment shaped primarily by the meltwater from retreating glaciers where flowing water influenced erosion, transportation and deposition to create a remarkable outwash plain.
The terraces overlie the ancient bedrock formation from the Lewisian Gneiss Complex a collection of ancient banded metamorphic rocks located in northwest Scotland and the Outer Hebrides. Among the oldest rocks in Britain they date from around 3.0 - 1.7 billion years ago spanning the Archaean and Proterozoic eras. The complex is composed primarily of gneisses formed from original rocks like granodiorite, tonalite and basalt that were transformed and folded under intense heat and pressure from most notably the multiple tectonic events during the the Scourian (2.9 – 2.3 billion years ago) and Laxfordian tectonic event (1.9 – 1.6 billion years ago). The latter was driven by the collision and accretion of the ancient Rae and North Atlantic cratons and contributed significantly to the growth of early continental crust in the area.
The Torridonian Sandstone is a younger, reddish-brown sedimentary rock that lies on top of the much older Lewisian Gneiss Complex in northwest Scotland. Deposited between 1.2 billion - 800 million years ago it signifies a shift from deep crustal metamorphism to surface sedimentation. Formed from the erosion of a vast mountain range its sediments of sand, mud and pebbles were transported by braided rivers across an arid landscape dotted with temporary lakes and prone to flooding. This process took place in a tectonically active rift environment where crustal stretching created fault-bounded basins that accumulated thick sedimentary deposits. Over time, these sediments compacted and cemented forming rock layers up to 7.5 Km thick. The Torridonian Sandstone tells a story of ancient rivers flowing across the supercontinent Rodinia. Both the Lewisian Gneiss Complex and Torridonian Sandstone were not involved in the in the formation of the terraces.
The terraces themselves originated during the Loch Lomond Stadial also known as the Younger Dryas Stadial and Nahanagan Stadial in Ireland when glaciers dammed valleys such as Strath Bran creating large glacial lakes. This was the final cold phase of the British-Irish Ice Sheet occurring between approximately 12,900 - 11,700 years ago during the Late Pleistocene. It followed the warmer Windermere Interstadial and came after the Last Glacial Maximum marking a temporary but dramatic climatic reversal before the onset of the current Holocene interglacial.
Triggered by a sudden cooling event of up to 9°C this stadial event caused glaciers to re-expand particularly in upland Britain and Scotland. The most significant glacial formation during this period was the West Highland Glacier Complex centered on Rannoch Moor and Fort William. Additional, smaller ice fields and glaciers developed in the Cairngorms, Monadhliath, Northwest Highlands, and on the islands of Skye, Mull, Harris, and Arran. The stadial takes its name from Loch Lomond where well-preserved moraines provide evidence of the glacial advance.
This cold phase ended abruptly around 11,700 years ago when rapid warming caused the remaining ice masses to melt completely starting the Holocene epoch. The Loch Lomond Stadial represents a short-lived but significant return to glacial conditions just before the full retreat of ice from Britain and the beginning of the modern climate era.
Meltwater rivers from the retreating deposited layers of sand and gravel into these lakes forming deltaic sediment bodies. As glaciers retreated and lake levels dropped rivers eroded into these deposits through both hydraulic action (the force of water) and abrasion (the carrying of sediment) to producing the characteristic flat-topped, stepped terraces seen today.
Spanning roughly 230 hectares the Achnasheen Terraces are thought to contain an estimated 16 million tonnes of glaciofluvial sediment and offer a record of past meltwater environment and post-glacial terrain development during glacial retreat during late Quaternary Scotland.
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