Darwin at Glen Roy

Parallel Roads

Darwin studied the unique geology of Glen Roy when he returned from the Beagle voyage. The valley is noted for the geological puzzle of the three roads (“Parallel Roads”) – lake terraces that formed along the shorelines of an ancient ice-dammed lake. The lake existed during a brief period (some 900-1,100 years in duration) of climatic deterioration, during a much longer period of deglaciation, subsequent to the last main ice age (The Devensian). From a distance they resemble man-made roads running along the side of the Glen, hence the name.

Darwin made his “Gigantic Blunder” on his visit in June 1838 by drawing on his recent findings in South America during the Beagle expedition and believing that the shorelines were of marine origin. This was contradicted by Louis Agassiz’s Glacial theory of 1840 which postulated that the shorelines had been cut by freeze-thaw processes of lake ice during the maximum extent of glacial ice in the climatic reversal known as the Younger Dryas / Greenland Stadial 1 or locally the Loch Lomond Readvance. Four decades after his 1839 paper and shortly before his death, Darwin conceded that he was incorrect. Wikipedia writing ‘I do believe every word in my Glen Roy paper is false’.

It is now known that the feature is the remains of ancient shorelines formed at the end of the last ice age when an advancing glacier pushed up the water level of a lake that filled the valley.