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.
The Ice Age impacts on the Ancient River Gravels of Windyhills
What you see before you here is a shallow dell which, depending on the time of year, may or may not have a trickle of water running along it. As you walk around the woods at Windyhills, you will probably come across a number of these little valleys. They are very similar to each other, all being no more than a metre or two deep, show a relatively straight course, and lead across the white-stone gravels (deposited in an ancient river bed millions of years ago) in a generally south-easterly direction. If you follow the course of any of the dells towards the north-west, it will rise gently almost until you reach the northern side of the gravel ridge.
Why are these dells here, and why do they all lead across the Windyhills ridge rather than along the former river course marked out by those white stone gravels?
The answer to these questions relates to events during the last Ice Age, more than 15,000 years ago. The area around Windyhills has been covered by ice at least once (and probably many more times) during various glacial episodes over the past 800,000 years or so. At one stage during the last glaciation to affect the area, ice moving over the landscape from the north-west came to a halt more or less along the northern flank of the Windyhills gravel ridge. The position it halted at is revealed by the steep slope on that side of the ridge (see the illustrations in the panel on the information board). As the climate warmed towards the end of the ice age, more and more water was produced by melting of the glacier. That water was originally ponded up between the ice edge and the northward slope of the Windhills ridge, but as the ponds filled, eventually they would have spilled over the crest of the ridge, and the water then flowed across the gravels towards the ice-free lower ground to the south. As this water flowed across the gravels it cut away the pebbly ground to form various dells, such as the one in front of you just now. You are standing on the edge of a feature which has survived in this area for well over 10,000 years, a remnant of an ancient landscape.
Heatherbloom Pit
Some 40 million years ago, a river ran across this countryside, flowing past where you are presently standing on its way towards destination somewhere to the east. At that time the landscape of what is now the Buchan area would have looked totally different to now. Plants and animals would have been very different from those you are familiar with. Although the great dinosaurs had become extinct, the mammals which replaced them were only distant ancestors of present species.
Even more striking would have been the change in the shape of the land compared with the present day. You probably approached the site by walking over gently rising ground until you came to this quarry, but had you been standing here all those millions of years ago, you would have been on the floor of a valley, most likely surrounded by rolling hills, as illustrated in the first panel of this display board! The river would have been flowing swiftly over a bed of shiny white pebbles, exactly like the ones which you can see in the excavation in front of you. If you look closely, you will see that the pebbles are quite rounded – a sign of having been worn by flowing water. There are also layers of sand between the pebbles, marking periods when the flow of the river was slower. Many of these beds slope gently eastwards, suggesting that the ancient river flowed in that direction. The white pebbles are made of a very hard rock called quartzite. This is very similar to rock found around Durn Hill, just south of Portsoy on the Moray Firth coast. The ancient river flowed south-eastwards from the Durn Hill area to Windyhills, and then on towards what is now the North Sea, carrying these distinctive pebbles with it and dropping large number of them on its river-bed to form the materials you can see in the quarry.
What has happened to the hills, and why are river-bed gravels now positioned on high ground? The answer to this is illustrated in the second panel of the display board. From about 40 million years ago until about 10 million years ago, the climate of Scotland was warm and humid, almost tropical. Under such conditions many types of rock ‘rot’ – they disintegrate to form a sandy or clayey gravel. These rotted rocks are easily washed away by rain and river action, so the hills gradually were removed over the millennia. However, the layers of very hard quartzite pebbles laid down by the river on the former valley floor did not rot, but protected the ground beneath them, so that the rocks there did not get worn away. As the former valley sides lowered, so the valley floor stayed where it had been, until what had been low ground was left as the highest ground in the area, the position it is in at present (see panel 3 of the board). Therefore you are standing in a topsy-turvey landscape, where high ground has become low, and low ground high, all in the space of about 40 million years.