Local Geology Day 4
Notes on Geology and Topography of the Creag Choinnich Area
Geology
This map area has two main areas of high ground - the circular hill in the NW part after which the map is named, and the long ridge of Carn nan Sgliat that forms the SE boundary. The higher parts of both hills are underlain by granite but nearly all the rest of the area is carved out of a varied sequence of metamorphic rocks that were originally laid down as layers of sand, silt and carbonate-rich mud on the margins and deep floor of a long-vanished ocean.
This is the Creag Choinnich competition map with the geology superimposed (from the BGS Geology of Britain online viewer). Scattered outcrops of granite are shown in red. The other colours represent older highly deformed layers of Dalradian metamorphic rocks that were intruded by the granite magma over 400 million years ago. During the deformation of the Dalradian rocks, and before the granite magma was intruded, all the rocks to the east were pushed westwards, possibly for several tens of kilometres, over the top of the rock unit represented by the buff colour.
Although, as with the competition areas earlier in the week, granite is part of the Creag Choinnich story, this area has a much more varied geological tale to tell. This one map area is in effect a miniature version of the geology of much of this part of north-east Scotland. It is underlain by a series of what were originally sedimentary rock layers which have been squashed, folded, sheared, and intruded by a relatively small boss of granite. All this happened more than 420 million years ago when three continents were involved in a (very) slow-motion series of collisions. The sedimentary rocks that had been accumulating on the floor of the ocean that had separated the continents up to this point were pushed down into parts of the Earth’s crust where high temperatures and pressures turned them into metamorphic rocks. Sandstones became quartzites; mudstones and siltstones were turned into mica-rich schists; and limestones were transformed into metalimestones - or marble if they were very pure. As with all the other Royal Deeside 2017 competition areas, there is also lots of evidence of glacial and meltwater erosion as well as extensive sand and gravel deposition associated with the melting of the last ice sheet.
Colliding continents
Creag Choinnich is an excellent area to tease out some of the strands of the complex story about how Deeside’s rocks were formed. Many of the features and processes described below are common to most of Highland Scotland. Creag Choinnich’s geology is far from unique – but it does encompass many elements of the story in one compact area.
The photo below is a strong visual clue to the main process which affected all the rocks of the area. It demonstrates very beautifully what can happen to layers of rock caught between colliding continents. It shows the underside of one of the many metalimestone boulders scattered below the crags on the north side of Creag Choinnich. In the photo, there are clearly some layers that have weathered much more deeply than others. Acidic water has eaten into these carbonate-rich layers while other silty layers have weathered less readily and form harder projecting layers.
These crumpled rock layers graphically illustrate the compressive forces created by colliding continents (see text).
The deeply etched surface reveals the intricate fold structures created as the rock was slowly compressed in the jaws of the vice created by the approaching continents. What you can’t see in an individual boulder or outcrop is same type of crumpling on a regional scale. Whereas folds in the photo are measured in tens of centimetres, the entire local sequence of schists, quartzites, and metalimestones has been similarly and repeatedly crumpled on various scales measured in tens and hundreds of metres, and even up to a scale of tens of kilometres.
In some places, instead of buckling into folds under the relentless compressional forces, the rock layers ruptured, with one set sliding over the rest along a horizontal plane of weakness known as a thrust fault or boundary slide. A layer of mica schist that forms the lowest layer on the west side of Creag Choinnich includes just such a boundary slide. This same feature continues right along the foot of all the hills on the south side of the Dee valley westwards as far as and slightly beyond Mar Lodge. It is regionally a very significant geological feature since all the rocks above it became detached from the layers below and literally slid millimetre by millimetre over the top of them. The almost surreal consequence of this 470 million year old movement is that when you stand on any of the hills on the south side of the Dee valley from Creag Choinnich westwards, the metamorphic rocks under your feet actually arrived in their present location having been pushed horizontally several tens of kilometres from the south-east. The layer of mica schist formed a plane of least resistance and facilitated the ever-so-slow advance of the detached mass of rock ahead of an advancing continent behind it. This was no local phenomenon however. This detached mass of rock (known as a nappe) and several other similar ones are known to extend over distances that stretch right across Scotland to the west coast.
Looked at from below, in the car park or arena, the whole hill above you is made of rock which has advanced over the rock beneath your feet, having come perhaps from somewhere near where Kirriemuir is today. All of this happened many kilometres below the present-lay land surface however, and the evidence for this has only been revealed through the slow erosion of thick layers of overlying rock formations.
Quartzite – the survivor
In addition to the metalimestone described above, quartzite is another metamorphic rock which commonly forms the many boulders and crags on the Creag Choinnich map. Originally sand made up largely of individual quartz grains, this rock is now composed of these gains welded together with more quartz, a mineral that is both highly stable chemically and extremely hard physically. Because of this, quartzite is a ‘survivor’ that doesn’t get broken down much during weathering, erosion and transport, processes that reduce many other rocks to mud and clay. It features prominently in the recently greatly increased exposure of river bank boulders and pebbles along the course of the River Dee. Quartzite dominates among the pebbles on Aberdeen beach too, where much of what is washed down the Dee lands up.
Typical outcrop of quartzite near Braemar, showing original layers that existed in the sandstone parent rock prior to metamorphism.
Topography
Like Birsemore, the northern slopes of Creag Choinnich hill have been cut back by the flow of ice in the main Dee valley, leaving a raw and rocky steep north-facing slope with ramparts of crags and sheets of scree. In marked contrast, the southern and western slopes are blanketed in deposits of sand and gravel with mounds and hollows creating interesting topographic variety. Bedrock is close to the surface over much of the other half of the map to the south and east. Here the topographic detail has been carved out by grinding ice and torrential flows of melt-water.
Peter Craig, July 2017