Geodiversity Around Portencross Castle EarthCache

The UK is a geodiversity world hot-spot and the environs around Portencross Castle are a local particular hot-spot. The purpose of this EarthCache is to show you some of this geodiversity. Even if you don't do the cache, it's a nice, and very popular, walk.

The Strathclyde RIGS Group (Regionally Important Geological Sites Group) surveyed the area and their work and the resultant leaflet produced by The Friends of Portencross Castle is the inspiration for this EarthCache. RIGS are landscapes or rock features that are valuable for education and tourism. Scottish Natural Heritage and The Geological Society of Glasgow supported The Strathclyde RIGS Group in its study.

1. Sedimentary rocks
   • sandstone
   • mudstone
   • conglomerates
2. Igneous rocks
   • dolerite
   • granite
3. Intrusive dykes
4. Erosion
5. Folds and faults
6. Post glacial rebound
7. Comparative weathering

Other EarthCaches go into the subject matters much deeper, the point of this EarthCache is the local geodiversity rather than any particular feature. The given coordinates are for Portencross Castle, but the best place to start the EarthCache is at the car park, as noted below. At the time of publication, the car park is free. I advise that you do not attempt to park elsewhere. Should you wish to go into the castle (highly recommended, but not required for this EarthCache), the castle is only open during the summer months. Entrance is free, although there is a box for donations.

Look at the rock formation between the car park and the sea. This is mostly brown sandstone known by geologists as Lower Old Red Sandstone. Sandstone is a sedimentary rock, which means that the rock is formed by the deposition and solidification of grains by water, ice or wind. Sedimentary rocks are always formed in layers or strata. Because they are formed by deposition, at the time of formation the top surface is usually more or less horizontal. However, because of internal stresses in the earth, sometimes the strata fold and in front of the car park you can clearly see the result of these folds and faults.

On the shore just south of the car park you can see where cracks in the sandstone have been filled with grey/green mudstone. Mudstone is a very finely grained sedimentary rock and consists of a mixture of clay and quartz silt particles. Other rock types formed in a similar manner are clay, shale, slate and siltstone. The character and hardness of these stones varies enormously according to their geological age and tectonic activity.

Walk north from the car park and castle and just after the old concrete Portencross Pier, a cutting has been blasted through the rock, known locally as The Throughlet. The ridge through which the path has been cut is known as an intrusive dyke. Around 300 million years ago volcanic activity forced molten lava through cracks in the existing sandstone and this then slowly cooled and solidified into an igneous rock called dolerite. (Igneous just means a rock formed from molten lava and intrusive means that it was forced along rather than being deposited as sediment). You will be able to see its crystalline structure, the crystals forming as it cooled. What defines a dyke is that it cuts across the bedding planes of the rock it intrudes. When an intrusion cuts along the bedding planes, it is called a sill. In a simple set of flat-lying rock beds, dykes are vertical and sills are horizontal, but in tilted and folded rocks, as is the case here, dykes and sills may be tilted too.

Dolerite is a much harder material than sandstone and is much more resistant to wear. Over the years the sandstone has worn down much more than the dolerite and so the dyke has become a ridge on the landscape.

There is a particularly large concentration of dykes on the not too far away Isle of Arran.

You do not need to visit these coordinates, the feature can be seen from the track.

Immediately north of The Throughlet, you can see an area of flat ground leading up to the cliffs. The cliffs used to be sea cliffs and the flat ground was the beach. During the last Ice Age, the huge weight of ice that covered the ground caused the ground to sink and, now that the ice has melted, the ground is still rebounding. Generally in Great Britain, Scotland and the most northerly part of England is still rising and the rest is sinking. As the ground rises, so the relative sea level falls and so the old beach is now high and dry. In this part of the world, the relative sea level has dropped around 10 metres over the last 10,000 years. Posh geologists call this phenomenon post glacial rebound or glacial isostasy. (Technically we are still in an Ice Age, despite global warming, and we are now in an interglacial period of a continuing Ice Age, but ignoring that scientific technicality, the ice sheet finally melted here about 10,000 years ago.)

Question 4. Approximately what is the distance from the track to the cliffs at the widest point of the raised beach?

Question 5. Although the rate of rise is not constant, what has been the average rate of rise per year over the last 10,000 years?

4. The Inlet N55 42.365 W004 54.247

Walk northwards until you are close to the above coordinates, about level with the white cottage by the cliffs. During this walk you will have walked past The Inlet, a cutting into the land by the sea. Immediately south of The Inlet the rock looks similar to that by the car park, except that the slope of the folds is steeper. Immediately after The Inlet, the slope is much less and when you are level with the white cottage there is almost no slope at all. A crack or fault caused this change. You have just walked over, at The Inlet, a 30 million year time-line. The rock north of The Inlet is a conglomerate called puddingstone and you can clearly see that it is a mix of gravel, cobbles and even the occasional boulder embedded in the sandstone. Puddingstone is a name given to a conglomerate in which the embedded stones are distinctly rounded. Those who gave it its name thought that the structure resembled a fruit pudding, with the stones the fruit and the sandstone the suet. The puddingstone here is thought to be about 380 million years old.

Question 6. If possible, but it's not compulsory, take a photo of the puddingstone with your GPSr, hand or whatever in the shot to show that it’s your own photo of the puddingstone. Don’t email it to me, upload it with your log.

Question 7. How do you think that the fruit got into the pudding and why is it rounded?

5. The Castle N55 41.960 W004 54.296

On the way back to the car park, call by the castle. Mostly the castle is made from locally hewn red/brown sandstone, although the blocks at the corners are of a much lighter coloured sandstone brought from a quarry at Kaim Hill, about 2 miles away. You can also see some blocks of Ross of Mull granite, with a pinkish smooth surface. Granite is the most common igneous rock found on the earth’s surface. The most severe weathering or erosion, is evident on the lea side of the castle and best seen on the walls around and adjacent to the main entrance (remember though that the castle has been substantially renovated.) The difference in erosion between the least (igneous) and most (sedimentary sandstone) weathered blocks is remarkable. After a period of stormy weather, significant amounts of grains of sandstone can be seen on the path to the front door – unless one of the volunteer guides has swept up!

Question 8. How much less, in inches or mm, are the least weathered stones eroded compared to the most weathered?

To claim a find, email me your answers via my profile page or message link at the top of this page. There is no need to wait for a reply before logging. If there is something drastically wrong with your answers, I'll reply to let you know and if they're OK, I'll also try to let you know.