The aim of this report is to define the geological evolution of the area around Stirling University and the Bridge of Allan.

36E3: Earth & Landscape Evolution Autumn Semester 2003

Student No. 1117083

Background:

Introduction

This report is produced in conjunction with the University of Stirling Environmental Science course 36E3.

The aim of this report is to define the geological evolution of the area around Stirling University and the Bridge of Allan. In order that this be fully understood and correctly researched, a look at the bigger picture of Scottish geological evolution has been necessary as well as looking at the geology of the local area in closer detail.The geographical area covered by this report is bounded by OS co-ordinates:

Southwest: 278000,695000

Northeast: 283000, 699000

Figure 1.0 shows this area in detail, using a 1:50,000 Ordinance Survey Map Extract.

To enable us to fully understand the geology of the area, site visits were made to three locations within the bounded area of the report.

Site 1:

Wolf’s Hole Quarry (Fig 2.0)

NGR NS 7896 9808

Site 2:

Hermitage Woods (Fig 3.0

NGR NS 8120 9681

Site 3:

Hermitage Woods (Fig 3.0)

NGR NS 8115 9676

(Fig 1.0)

(Fig 2.0)

(Fig 3.0)

Geological History of Area:

History

‘Stirling area sits astride a major boundary between two blocks, brought together between 450 and 420 million years ago. The highlands and Lowlands are separated by the near vertical Highland Boundary Fault, a large fracture that penetrates deep into the earth’s crust separating different crustal blocks.’ (British Geological Survey: Loch Lomond to Stirling).

The British Geological Survey of the Stirling Area as quoted describes briefly the geological features of the area.

Within Scotland, and particularly the central belt of Scotland where the Lowlands dramatically meet the Highlands, there lies some of the most extraordinary and miss-matched Geology seen anywhere on the planet, this can be explained by looking at the history of the overall evolution of Scotland. The geological structure of Scotland can be explained as an amalgamation of various fragments of the Earth’s tectonic plates.

The geological area encompassed by this report is located within an area that is known as the Midland Valley, and lies between the Highland Boundary Fault and the Southern Upland Fault (Fig 4.0). Some of the oldest rocks in this Midland Valley region date from the Ordovician period, around 470 million years ago. At that time northern Scotland lay at the southern edge of a continent known as Laurentia with the area that was to become the Midland Valley, forming a line of island volcanoes in the adjoining and gradually closing Iapetus Ocean. The Iapetus Ocean closed during Silurian times, 435 million years ago, in a continental collision known as the Caledonian Orogeny.

During the Caledonian Orogeny plate tectonics caused the continents to move towards each other, closing the ocean. During this time, the ocean floor was subducted beneath Laurentia. On the final closure of the Iapetus Ocean, the continents collided and a period of mountain building took place. This joined the crustal foundations of both Scotland and England. Erosion of the mountains to the north and south, produced sand, silt and mud that was carried in to the Midland Valley area, covering the remains of the volcanic island chain. By Devonian and early Carboniferous times, Scotland lay just south of the equator. ...

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Geologist have determined through research that the area in question is the result of mountain building during the Devonian Period, where through the Caledonian Orogeny, the landscape of the Ochil hills surrounding Stirling, although weathered, is as we see it today. (Fig 5.0) The Ochil Hills are formed from the Andesitic and basaltic lava flows from volcanic activity in the Devonian to early Carboniferous period of 415 to 345 million years ago. The lava was embedded with conglomerates and air fall ash deposits. This structure sits juxtaposed to the much softer and relatively younger carboniferous sediments which lie directly to the south of the fault. In comparison with this sediment which can be seen to the south, the Ochil Hills themselves have resisted erosion particularly well, as some of the top layers of sediment to the south have been sorted and smoothed as a direct result of various occurrences in recent geological times.

Fig 5.0

Sand, silt and mud the product of erosion of the mountains in the North and South was carried into the Midland Valley by rivers and through the rock cycle was deposited. The deposition of these clastic sediments and subsequent metamorphism allows us to arrange in a specific order, a picture of geological time in the creation of the area of this report.

Through the use of sampling at different sites it is possible to look at the geological structure of the area in question.

Site 1 – Wolf’s Hole Quarry

This site is a disused quarry with a large visible outcrop that consists of two different rock types. (Fig 6.0)

The lower band of rock is sedimentary sandstone that has a fine / medium grain size and a pinkish colour. This sandstone has a vertical blocky stratum leading to the plane of unconformity. The sporadic pinkish colouring to the sandstone could indicate the presence of iron oxides. The upper layer of rock is unfortunately inaccessible but we are able to tell that it is a basaltic lava flow from small samples on the quarry floor. The lower band of sandstone dips at an angle of approximately 20o in a north westerly direction. (Fig 8.0). From the understanding ascertained by researching geological maps, and through reading, we can say that it is part of the Garvock Group. This group is made of two sections, known as the lower Sheriffmuir, and upper, Dunblane Formations.

These formations can be correlated to be the area in which Wolf’s Hole Quarry is located as these formations consist of brown or grey and red fluvial sandstones. The sandstone in the quarry is a mixture of these two types of sandstone.

Fig 8.0

From the sampling at Site 1, we can show that within this area large deposits of sand were deposited and were then subsequently over a period of time metamorphosed into sandstone through the presence of pressure and heat. In this Devonian age, Scotland was situated, as mentioned earlier, slightly to the south of the equator and the climate was defined as semi-arid. A large, subsiding basin was created due to the extension (north and south) of the crust. Large amounts of sediments were moved from the Caledonian mountains into this system by very large river systems, comparable in size to the Mississippi river system in the United States of America – the sedimentary rocks formed by these processes were mainly fluvial sandstones.

These fluvial sandstones can be seen in Wolf’s Hole Quarry. The large blocks of sandstone, with very fine, small to medium grain sized particles is representative of the amount of energy present in its formation, indicative that the sandstone was formed as a result of contact metamorphism in an ephemeral lake environment. The blocks also have smaller unconformities between them – which points to erosion between periods of bigger geological events.

This rock is overlaid by a horizontal layer of Basaltic and Andesitic lavas – these are separated from the sandstone by a very clearly defined line of unconformity as illustrated in the photograph below. There is no sign of melting of rock at the plane of unconformity. This would therefore show that there had been a period of erosion before the basalt lava flow which swept clean the surface of the sandstone. This would equate to the medium grained clastic matter found within the lava samples. This also shows that the sandstone is Devonian in period and the Lava flow Carboniferous.

At the second and third sites the construction of the rock shows a different story. There is in this area large conglomerate. (Fig 9.0). This would give evidence of a river flowing through the three sites with large boulders, cobbles and pebbles being deposited in the area of Hermitage woods and the small sands and silts in the area of Wolf’s Hole Quarry.

Fig 9.0

The large conglomerates found at Hermitage Woods, must have been laid down in an environment of high energy. The bulbous shapes within the conglomerate at this site are an indication of lava flow. There is also within this site some small evidence of freeze cracking. The photo below also shows that the rock in this area also follows the same dipping pattern to that visible at Wolf’s Hole Quarry.

Using a geological map of the area we can show the underlying bedrock and with the use of cross sectional drawings show the lie of the rock and the direction of dip of the sandstone at location 1 – Wolf’s Hole Quarry.

These geological maps also show the main fault lines in the area allowing us to determine the effect of faulting on the rocks in the three locations.

Fig 10.0 Geological Solid Map of Stirling Bridge of Allan Area.

Site 1

Site 2

Site 3

Fig 10.0 shows the geological solid features of the Stirling and Bridge of Allan Area. Clearly marked are the locations of the three field sites and also line AB shown below as a cross section. (See Solid Geology Map in Annex.)

Site 1 Site 2 & 3

Looking at Fig 5.0 we can see the effect of faulting on the report area. The relative age of the rocks within the report area as discovered at the three field sites show that there has been down throw to the southern extreme of the area. This down throw indicates the movement of rock to the north uplifting many metres. The topography of this area also indicates that the rocks to the north (Devonian Lavas)) are harder than those to the south (Carboniferous Sedimentary).

All of the evidence of faulting and rock age is supported by the dating of samples taken at Wolf’s Hole Quarry and Hermitage Woods.

The southern planes stretching away from the Ochil Hills with their softer sedimentary rocks has allowed for greater industrial and domestic usage. Evidence of this can be seen from the photograph below.

The softness of the rocks in this lower plane is evident from the movement of the river’s meandering and subsequent cutting and erosion of the land. To the north of this plane land use is predominantly that of highland farming with industry being very minor if at all. The lower plane’s sedimentary rocks have over hundreds of years been manipulated by the inhabitants of it. In more recent times this has been primarily through the mining of coal.

There is also a critical change in transport routes along both north and south planes. It is very easy for travel in an East West direction south of the Ochil Hills, something that a few miles north is not possible easily. Within the lower plane ice has also played a significant factor in the shaping of the landscape. This lower plane of softer rock with the occasional volcanic feature has been largely swept smooth by moving ice flows. As ice has met harder rock it has formed crag and tail features in an east west alignment. These high points in the lower planes are where large settlements can be found and through history have been used for such a purpose with heraldic seats being taken in castles built on these crag and tail features, most notably Edinburgh and Stirling Castles.

Conclusions

The area bounded by this report contains some of the most diverse and complicated geology to be found within Scotland. It is possible from research carried out through field studies and also reading to look at the area from its conception and subsequent changes over 400 million years.

It is not easy to visual Scotland in today’s climate ever being a tropical land south of the equator but by piecing together historical and geological data this is possible. The rock formations in and around Stirling, show evidence that they were formed during periods of high volcanic activity and that plate movements were also extremely active. The stark contrast in hard and soft rock North and South of the Ochil fault line allow for such a dramatic change in the topography and geology of the area.

Bibliography

Background reading

Bennison, G., (2003) An Introduction to Geological Structures & Maps. Oxford University Press. London
Corbett, L., et al. (1993) Central Scotland: Land-Wildlife-People. Forth Naturalist and Historian.
SNH/BGS Loch Lomond to Stirling: A Landscape Fashioned by Geology.