The extensive braid bars of middle Glen Feshie
Simulation of channel patterns in Glen Feshie at different discharges. Image courtesy of the University of Cambridge.
Glen Feshie is a Site of Special Scientific interest (Werritty and McEwen, 1993) for the excellence of its river landforms.
1. Braided channel systems
The broad floor of the Feshie above the Lodge presents many features of a gravel bed river. Indeed, it probably represents the closest analogue that we have today to the braided sandar in front of glaciers like those in Iceland today.
Braiding refers to the way in which the river channel continually branches. The channels move between braid bars and often stand at different levels. These height differences of up to 1m mean that channel switches are common. The braid bars are sigmoidal in shape and the clasts on the bars are imbricate and become smaller towards the downstream end of the bar. The rapidity of bar erosion and channel migration can be judged from the lack of vegetation on many bars.
2. Debris cones
Below Craig na Caillich, the Feshie has cut a fine section through a series of debris cones (Brazier and Ballantyne, 1989). The exposed sediment is poorly sorted and comprises angular clasts of schist in a matrix of coarse sand in sheets up to 1 m thick These are debris flow deposits, produced when slurries of saturated debris flowed down the gulleys during intense rainfall. The interest of the section is enhanced by the layers of organic debris and soil. Radiocarbon dates show that this material has accumulated over the last 200 years. Earlier debris cones may well have existed on this site but have been lost to erosion by the Feshie.
3. River terraces
Five sets of terraces are recognised. The highest carries enclosed hollows formed when buried blocks of glacier ice melted out (kettle holes) and clearly dates to the deglaciation of the glen around 13,000 years ago. Terraces at lower levels (5m, 3m and 1.5 m above the present river) date from the Holocene (10ka, 3.6 ka and 1 ka, respectively) and a mid 19th century terrace is also developed locally. The discharges reconstructed for the 3.6 ka terrace suggest a level twice that of the average for the present Feshie (Robertson-Rintoul, 1986).
4. Alluvial fan
junction of the Feshie with the Spey is marked by a huge alluvial fan consisting
of gravel carried down the Feshie and which the Spey has proved incapable of
shifting, leading to a marked construction of the valley floor downstream of
Loch Insh. The largest part of the fan dates from the Lateglacial and represents
debris transported by melting glaciers and by snow melt during the Loch Lomond
Stadial. The fan remains highly active today, with extensive gravel bars at low
water and evidence of recent channel switching (Brazier and Werritty, 1994).