Geology and soils

The basement rocks consist of ancient Pre-Cambrian metamorphosed sediments of the Moine series (dominated by quartz-mica schists) and, to the west, the Dalradian series bringing mixed acid-basic soils with some limestone. Two groups of igneous rocks were then intruded during the Caledonian orogeny. The granite group forms the spine of mountains which flank the valley from the central Cairngorms, to the plateau of Beinn a’Bhuird-Ben Avon and the Whilte Mounth to Mount Keen and the Hill of Fare. Granite mountains have the characteristic cone-top shapes of Mount Keen and Meikle Pap, or huge summit boulders (tors), such as on Clachnaben.  Another igneous group of base rich rocks includes gabbro and hornblende schists around Morven and serpentine around Muick.

The soils have developed over the last 10,000 years from the extensive morraines which fill the length of the valley and reflect the intense and complex glaciation of NE Scotland. These deposits include tills, erratics, fine lacustrine (lake) deposits and the coarse fluvio-glacial sands and gravels which are quarried extensively around Drumoak. Different tills impart the certain qualities of texture and chemistry to the soils depending on the parent rocks, but the tills may have travelled up to a mile from the rocks of origin.

In the mountains, thin soils and outcropping rock impart little to the chemistry of the rainwater running into the River Dee headwaters and allow fast runoff during storms. On the granitic moorlands extensive areas of peats (>5 m in Glen Dye and other places) and humic iron podzols leach carbon into the stream, accounting for the dark colouration during storms.

In the middle catchment thick glacial deposits, along with deeply fissured granite (from past exposure to a tropical climate in the Tertiary period) give subterranean water storage that maintains summer flows. Generally, these slowly weathering soils have limited buffering capacity against atmospheric pollution and natural soil acidity. Hence, the ecosystem of the River Dee is adapted to the natural, low levels of nutrients and susceptible to nutrient and other pollutant inputs from human activity.

Agriculture is focussed on soils with favourable characteristics; improved grazing on limestone areas in the western uplands, intensified agriculture halfway along the River Dee on the basic igneous rocks around Tarland and the cereal production on the flat alluvial plains bordering the lowland river. A long history of agricultural preparation has led to the clearing of erratics (glacially-transported boulders) giving the characteristic piles of granite rocks in the corner of every field, drystone walls and the Kingswell west consumption dyke, an 8.2 m by 1.2 m and 457 m long structure built to ‘consume’ rocks.

For further information; Northeast Scotland. A landscape fashioned by geology.

 

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