Around 300 million years ago the tectonic plates beneath what is now Central Scotland were stretching and splitting apart. This tension caused the Ochil Fault to develop (see GC4AQR3 ) and also volcanic activity. One result of this was the intrusion of molten magma underground, using lines of weakness to force its way between sedimentary layers of rock. The magma cooled to form a horizontal sheet of igneous rock which we call a sill. This Midland Valley Sill can be found under much of the Forth estuary, outcropping at the edges of the Carboniferous basin into which it was intruded. As well as the outcrops in the Stirling Area, it can be seen on the Lomond Hills in Fife (GC251QC and GC1PPQJ) and at North Queensferry (GC451G3). Unlike Edinburgh Castle, Stirling Castle does not sit on the eroded plug of an extinct volcano - the Stirling rock is part of the Sill.
The composition of the Sill is quartz dolerite, a hard medium-grained igneous rock. Because the magma cooled underground, the crystals had time to grow to a size visible with a hand lens. They did not grow as slowly as when deep-seated magma cools, which allows large crystals to form, easily seen with the naked eye.
The rock of the Stirling outcrop was used to build the town walls and the fortress part of the Castle. Because the dolerite is so hard, other building stone, especally sandstone, was used for the residential appartments, where it could be carved to provide decoration.
Parking is possible on the Castle Esplanade but this will cost £4 at the time of writing. A preferable parking spot is along Upper Castlehill, where it is free.
This earthcache will take you on a short walk both to see the quartz dolerite of the Castle outcrop at close hand and also to see across to some of the other exposed Sill remnants. As you go, please answer the relevant questions and then e-mail me the answers. Do not put the answers in your log. You may log once you have done this.
Cache co-ordinates: Upper Castlehill. Here you can see the dolerite outcropping naturally. You can also see part of the Castle wall built onto it. Take notice of what the dolerite looks like, both as part of the hill and in the wall.
Make your way up the pathway and steps to the Castle esplanade.
Waypoint 1: Look out north-east towards the Wallace Monument.
Question: Estimate the height of the Abbey Craig outcrop compared with where you stand. Is it higher, lower or about the same?
Now cross the esplanade.
Waypoint 2: Look out south-west towards Gillies Hill. It is the tree-covered hill in the middle distance with some very tall Giant Redwoods sticking up way above the rest. Behind this you can see the slightly higher forest-covered slopes, with a nick in them, which lead to Lewis Hill above the North Third reservoir (not visible). Close at hand you can see the dolerite outcrops in the Drummond pleasure ground and the cemetery just beyond.
Question: Can you think of a reason for the present-day isolated nature of these outcrops when they were once all part of a continuous sheet, the Sill?
Now go down the steps at the south end of the esplanade to the old grammar school, now the Portcullis Hotel. This is a very interesting building with both dolerite and sandstone used in its constuction.
Waypoint 3: Round the back of the Hotel you can see part of the wall of the Castle complex next to one wall of the Portcullis Hotel.
Question: Describe the differences you can see between the stone used for the two at this point.
Waypoint 4: (Optional) If you like you can climb up the small dolerite outcrop to a direction indicator showing the history of the area. From here there is a good view to the north-west to the wooded hill of Craigforth, as well as the view across to Gillies Hill.
Waypoint 5: (Optional) This is a viewpoint on the Back Walk. Following the Back Walk is an excellent way to see the dolerite in the Old Town Walls, to see many outcrops and to experience the elevated nature of the Castle crags. If you follow it around the north of the Castle it will bring you out onto the Ballengeich Pass and back to the parkng.
We hope you enjoyed your visit to Silly Stirling.