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PoP Feature: Past of Perthshire - Perth: Scenic Setting and Evolution - Perthshire Chamber of Commerce

PoP Feature: Past of Perthshire - Perth: Scenic Setting and Evolution

To help share some of the key historical moments in Perth’s history Perthshire Chamber of Commerce and the Royal Scottish Geographical Society have joined forces to produce a timeline of Perth over the ages, beginning with the geological formation of Strathtay and working its way (over the next few months) through funny, interesting or critical historical moments to give a flavour of this Fair city, its royal, social, political and industrial heritage and what makes it the City, (and makes Perthshire and Kinross the area) it is today.

Scotland, as we know it now, began life as part of a huge and mountainous landmass, the sand of which formed the sandstone basis of much of our current local geology.  Lava welling up through this sandstone formed the basis of many of our hills such as Kinnoull and the Sidlaws which in effect formed the northern escarpment of a rift valley with the Tay at its centre.  This is the early history of the Strathtay and the bedrock of Perth and its surroundings.

                                      Perth: Scenic Setting and Evolution
 
Introduction
A city is the product of its geography and history. The maps and diagrams aim to give you some notion of the evolution of the physical landscape of Perth and it surrounding area.


Fig 1: Physiographic Sketch of Perth’s Site and Setting
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Perth: Geological and Scenic Context
 
Fig.1 illustrates the physical setting of Perth in a simplified way. It shows the division between “Highland” and “Lowland” Perthshire, separated by the prominent abrupt face of the Highland Boundary Fault (H.B.F.). The HBF is clearly oriented from north-east to south-west, a characteristic trend known as the Caledonian trend.  Breaching the H.B.F. are the transverse valleys of the Almond, the Middle Tay and the Ericht rivers.

Highland” Perthshire is the southern part of the Grampian Highlands among whose significant peaks are Perthshire’s two highest: Ben Lawers (1214 m.) and Schiehallion (1083 m.) These are the worn-down, deeply eroded stumps of a great mountain belt composed of hard, metamorphic, crystalline rocks created during massive plate tectonic collisions called the Caledonian Orogeny, dating between c.520 to 400 million years ago.
“Lowland” Perthshire lies south of the H.B.F.; it is geologically younger and is a northern part of the Scottish Midland Valley. Its landscape is that of Devonian (also called the Old Red Sandstone) Period lasting from 400 to 350 million years ago, and has two dominant elements of relief.

  • Firstly, sedimentary basins of relatively soft red sandstones, especially  Strathmore (Great Valley), the extensive fertile lowland which stretches north-eastwards from Dunblane to the North Sea at Montrose, the lower valley of the River Earn, and the Carse of Gowrie.
  • Secondly, and formed at the same time - though with massive explosive force - hard volcanic uplands with their steep, river-facing slopes: the Ochil Hills, south of the Tay Estuary and the Sidlaw Hills, including Kinnoull Hill (222m) and Moncreiffe Hill (223m) to the north.

 
Fig 2: Simplified Geology Map of Perth
 
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Fig. 2 is a simplified map of the geology of Perth. How it developed is a complex story and the following diagrams and text simplify the evolution of the geology and landforms of Perth and the surrounding area.  
 
Perth: Evolving Geology and Scenery c. 500-450 million years ago
 
A critical period in Perth’s geological history occurred during the Devonian-ORS Period (c. 500-450 million years ago) and is summarised below. It is important to remember: (i) that there was a great deal of tectonic activity. Plate movements throughout these years meant that the Perth area was very unstable, as demonstrated by volcanic activity, earthquakes, folding and faulting of rocks; and (ii) that Scotland lay in the sub-tropical latitudes of the southern hemisphere.
 
“Scotland”, during these years was part of a huge arid, mountainous landmass called the Old Red Sandstone Continent, shown below in fig.3.  

Fig 3: Old Red Sandstone Continent by 400 million years ago
 
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The map shows that the ORS Continent consisted of North America and Greenland, and that Scotland then lay south of the equator. This resulted in desert conditions in Scotland. Not only was the climate hotter and arid, but occasional heavy bursts of rainfall caused devastating flash-floods. Ephemeral but frequently torrential rivers and streams rapidly swept huge bed-loads of sediment - red coloured, iron-rich sand, pebbles, gravel and boulders - from the surrounding, vegetation-free mountains into large, land - locked lakes as shown below in Fig. 6. At the same time as these sediments were accumulating, vigorous volcanic activity was taking place. Thick piles of lava and localised zones of ash accumulated, often interbedded with the sedimentary sandstones. These tough lavas, mainly composed of basalt and andesite, form the Sidlaw and the Ochil Hills.   


Fig 4: About 395-390 Million years ago
 
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By about 370 million years ago, the laying down of sedimentary rocks (sedimentation) and volcanic activity had come to an end. This marked the end of what geologists call the Lower Devonian/ORS age. By then new earth pressures [from about 387-374 million years ago] had resulted in large scale folds which followed the north-east to south-west ‘grain‘ of the country. Fig. 5 shows (i) that beneath what is today the fertile, level land of Strathmore the rocks now formed a large, deep downfold of sedimentary rocks called the Strathmore Syncline; and (ii) to the south of it lay an upfold or arch of lava and ash called the Sidlaw-Ochil Anticline. During this new age called the Upper Devonian/ORS, new fresh sediments - red sands and muds - were deposited from the upfolded Sidlaws and Ochils.
 
Fig 5: About 370 Million years ago
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After the deposition of the Upper Devonian-ORS, a sustained period of faulting took place. Resulting from such tectonic activity was the formation of the Tay Rift Valley. Fig. 6 shows the downward displacement by a depth of some 600 metres of the newer sedimentary rocks between (i) the North Tay Fault along the edge of the Sidlaws, best seen in the steep south-facing scarp edge of Kinnoull Hill and (ii), lying parallel to it, the South Tay Fault marking the northern edge of the Ochil Hills. A similar steep, fault-guided scarp marks the southern cliff face of Moncreiffe Hill.
Fig 6: About 359 Million years ago
 
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As a consequence of the lowering of the Upper Devonian/ORS sediments, the lower Tay Valley, between the Sidlaws and the Ochils is a fine exemplar of a rift valley, estimated by one academic as the ’best example of a true rift valley in Scotland, if not in the whole of Britain’ (J.B. Whittow 1977, p. 136: Geology and Scenery in Scotland)
 
At a later geological period (Carboniferous), a series of dykes (vertical walls of lava) were formed in the Sidlaws and the Ochils (see Fig.6) as lava intruded between layers of sedimentary rock. Examples of a dolerite dyke can be seen in the Corsiehill Quarry car park and also undergird the west-east trending Findo-Gask ridge to the west of Perth (see Fig. 1).
 
Quaternary Era - Shaping the landscape with Ice
In common with the whole of the country, the Perth area experienced several glaciations, peaking most recently about 18,000 years ago. By about 10, 000 years ago Perthshire was ice free. At different times during these Ice Age years, ice advanced eastwards (i) as ice sheets from centres of ice accumulation such as the huge basin of Rannoch Moor, and later, (ii) as valley glaciers, some up to 2 km in depth, from corries whose host mountains can be viewed  from vantage points on top of Kinnoull and Moncreiffe Hills

The consequences for the landscapes included: a deepening and reshaping of pre-existing valleys such as the Lower Tay and Earn; the deposition of thick blankets of glacial till [a melange of clay, silt, sand and stone], sometimes forming ice-moulded features such as drumlins in the lower Tay and Almond Valleys; and, as the ice decayed, melt-waters dumped extensive spreads of fluvio-glacial sands and gravels. Sometimes such deposits formed kames and eskers eg between Stanley and Luncarty, and were exploited for building purposes. Elsewhere, fluvio-glacial sands and gravels, eg at Barnhill and Kinfauns, were drained directly from the margins of the ice into a late-Glacial  sea then flooding the lower Almond, Tay and Earn valleys as climate began to ameliorate and sea level rose.

As the ice finally cleared the Tay and the Earn valleys, and the Carse of Gowrie, sea level was approximately 40 m higher than at present. Indeed, it has been suggested that, about 15,000 years , the Tay and the Earn valleys “were fiords initially with icebergs and then as the glaciers withered, only with winter ice” (D. Strachan 2010, in Carpow in Context p.18). In this cold estuarine sea, summer icebergs deposited  clays and silts forming a layer of carse clay which, along with later marine and peat deposits, covered what would emerge as the Carse of Gowrie as the land gradually rebounded in response to the removal of the former huge burden of ice.
 
Perhaps a final point is that although the Hills of Perth seem geologically static and stable, they are still experiencing geological and geomorphologic forces nowadays, though with reduced vigour. Weathering, both physical, chemical and biological occurs; landslides can develop on steeper slopes; streams feed into the larger Earn and Tay river systems - the Tay has the largest discharge of water in the UK with a stronger flow than the Severn  and Thames combined; and every so often there are reminders of tectonic activity along the Highland Boundary Fault with slight, non-life threatening earth tremors, especially near Comrie- the “Shakey Toun” with its Earthquake House and early seismographic instruments. Nor should it be forgotten that human actions also impact and alter the landscape.

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