Raised beaches are quite a common post-glacial geological feature in Scotland. There are two raised beaches here. Together they form the road and a grassy bank sloping down to the seaside promenade. It is a popular spot for dog-walking, kite-flying and cycling etc.. Few of those so engaged realize that the spot they are standing on would have been under quite a depth of water a mere few thousand years ago; a geological instant.
Up until approximately 15,000 years ago this area of Scotland was covered by a sheet of ice glaciers between 1 and 1.5 kilometers thick. The effects of the grinding action of the ice sheet can be seen in the local landscape. Visit the nearby Roche Moutonee earthcache to see the scratches left by the ice and the rocks carried within, and right in the centre of the City of Edinburgh, Castle rock and the Royal Mile comprise a classic ‘Crag and Tail’ erosive feature left visible after the ice retreated back towards the Highlands.
The grinding sandpaper-like action of the sediment-laden ice was not the only effect of the glacial sheets. They weighed billions of tonnes, so much that they pushed the land down beneath them. In fact at the end of the last main ice age the land was pushed down so far that the sea reached the bottom of Calton Hill, in the centre of Edinburgh, 35-40 meters above present sea level.
Locally the beach formed at this level can be found near the Silverknowes Golf Course club house, about 700 meters from here, up Silverknowes Road. It can be seen in the photograph below but is much easier to spot if you go to the site and look East from the club house. If you look out over the Forth from here it is thought-provoking to imagine the depth of water which would have covered the land in front of you at the end of the last ice age. The coordinates are given below, as a waypoint, for those who wish to visit it, but you do not have to go there to complete the cache.
When the ice melted the land level rose; a process called isostatic lift. The results of this can be seen here, and at many other sites along the coast of the Firth of Forth. Isostatic lift occurs in two main phases. First there is the elastic phase which occurs relatively quickly after the glaciers melt. Here this took place between about 15,000 and 13,000 years ago as the ice retreated westwards. The second is the slow viscous flow phase which has continued more slowly since then. This second phase is due to flow of the semi-liquid mantle under the Earth’s solid crust.
When the glaciers formed they pushed the crust down and the mantle flowed away from glaciated areas, pushing the crust up in surrounding regions. This surrounding raised area is called the forebulge. This includes Southern England and much of the floor of the North Sea.
Nearby at Cramond you can see an information board marking the site of one of the oldest settlements in Scotland dating from the Mesolithic period. The people who lived here, or their forebears, colonized Britain by walking over the land formed when the ice melted and part of the floor of the North Sea was above sea level so linking Britain to the Continent. If you walk up the River Almond from its mouth near the settlement site you will see the deep gorge that the river cut through the sandstone as isostatic lift took place.
The areas which were previously covered by thick ice sheets and glaciers are now rising as the mantle flow is reversed; a process called isostatic post-glacial readjustment. It is estimated that this is going to continue for another 10,000 years.
The map below shows areas of the UK and Ireland which are rising in green, those falling in red and areas of no change in orange. In the South of England the falling land levels have led to the formation of rias, flooded river valleys with a characteristic profile, such as the River Dart.
Image reproduced from Wikipedia. Author Kentynet.
So, the glaciers melt, the land rises relative to the sea after their weight is lifted, and the ancient beach ends up above modern sea level. Simple - right? Not quite. If that was the end of the story then the highest raised beach would actually be about 120 meters above modern sea level, the height the land has risen, but it isn’t, so why not?
What happens to all that ice from the glaciers? Well it melts and flows to the sea thereby raising the sea level, a process called eustatic sea level change. So the land level is much higher than it was 15,000 years ago, but so is the sea level, just not as high relative to the land level.
In fact various levels of raised beach can form at one location due to variations between land and sea levels during the process; isostatic lift and eustatic sea level change happen at different rates over time. At this site there are two raised beaches. Counterintuitively the higher one actually formed after the lower one.
The lower beach , about half way down the slope, formed during a mini ice age called the Loch Lomond Intersatadial, which lasted from 13,000 to about 11,500 years ago. Its level is marked by a clump of vegetation, beyond the young trees in the top photograph. During this period temperatures plummeted again, glaciers reformed in the Scottish Highlands reaching as far South as Loch Lomond, and sea levels dropped dramatically. When these glaciers melted sea levels again rose, overtaking the slowing isostatic lift and forming the higher beach here, which has been levelled to form the road.
8,000 years ago the sea level was at the height of the highest road, furthest from the sea. Since then the slow viscous flow phase of isostatic lift has continued causing relative sea level to drop to its current level.
Thanks to City of Edinburgh Parks Department for permission to site the cache.
To claim this cache you need to complete the following tasks.
1. Take a photograph of you and your GPS at the cache site with the raised beach in the background and post it with your log(optional).
2. Given that the density of a glacier is approximately 850 kilograms per cubic meter, what is the weight in kilograms of a glacier one kilometer thick by one kilometer wide and one kilometer deep?
3. Estimate the depth of water in meters that would have covered you if you were standing where the present high tide mark is 8,000 years ago i.e. the height of the top of the higher raised beach (road level), above the top of the modern beach.This is the height the land has risen over this period.
4. Using your answer from 3 now calculate the average annual rate of rise of the land here in milimeters per year.
5. Walk along the promenade to beyond the wood next to the slope. A waypoint for the location is given below (LLIBV). You can see the Loch Lomond interstadial beach as a marked change in the gradient of the land in the field in front of you. What is your estimate of the angle formed by the gently sloping land in front and the steeper part of the slope behind? What process do you think caused this marked change in gradient?
6. Visit the information board at the site of Cramond Mesolithic settlement (waypoint given below). How old is the settlement thought to be, and which two types of stone were the tools found there made from? Taking the local land gradients, age of the settlement, and the knowledge you now have about the changes in land and sea levels since its creation into account, how far away from the high tide mark do you think the settlement was when it was created? This part of the site is not wheelchair accessible. If you email me beforehand I can provide an alternative question for wheelchair users to prove you have visited the site.
Please email me your answers. Do not give any of the answers in your logs.
Congratulations to Crustyloafer for the FTF.