The following article and diagrams for the Windyhills Geological Interpretation were kindly provided by Dr. Alastair Gemmell of the School of Geosciences, University of Aberdeen.

Frozen Ground and the Windyhills Gravel Deposits

Looking around you, at this location you can see evidence of a number of stages of the development of the Windyhills Gravels, the ancient white pebbles that you can see in the quarry excavation faces both behind and in front of you. Behind you is the main gravel deposit. When this quarry face was fresh, you could see layers of the pebbles separated by thin nearly-horizontal layers of buff-white sand. What you saw there were the traces of sands and gravels dropped by an ancient river, which flowed, tens of millions of years ago, from the Banffshire area eastwards towards the North Sea.

In front of you is a similar set of sands and gravels, but some aspects of them can be seen more clearly than in the larger face behind you. Most of the pebbles (the white ones are quartzite, a very tough rock, while there may be a few pebbles of flint visible as well) are set in a sandy or silty soil. Notice that in the lower part of the face, the majority of the quartzite pebbles lie flat, almost horizontal, as in the illustration in the first panel on the board in front of you. If you look higher though, towards the top of the face, you will see that there are quite a few pebbles which have turned upwards, so that they are almost standing vertically in the face. This is a sign of prolonged frost action having affected the soil in the past, most probably towards the end of the last ice age some 12, 000 – 15, 000 years ago. Before the temperature dropped significantly, the pebbles would almost all have been lying horizontally. The area was covered by glacier ice as things got colder, but when the ice started to melt, it was still cold and frost would have been frequent and intense. As any water in the soil expands as it freezes, the result of a deep frost would be a bulging up the ground surface which caused pebbles to be dragged upwards with the ground surface. Each time the ground froze, pebbles near the surface got pulled upwards. At the same time they got turned so that they tended to sit vertically in the soil rather than lie horizontally (see panels 2 and 3 on the board).

The zone of upright stones marks that part of the soil which has been affected by frost action. The thickness of that zone below the surface is an indication of the intensity of past freezing of the ground (right hand panel in the illustration). The thicker the layer of upright stones, the more intensive the frost action has been. From the face in front of you, it can be seen that the area has experienced some pretty intensive freezing of the ground in the past. Thus the Windyhills gravels show evidence of extensive climate change affecting the Buchan area over millions of years, ranging from warm, humid, almost subtropical climates tens of millions of years ago to glaciation and intense ground freezing no more than 20,000 years ago. This little quarry has quite a story to tell.