Ancient Soils in the Culbin Forest
MFC Paper 03 by SM Ross 1975
In Bulletin No. 2 (1974) mention was made of relict soils capping some of the old inland shingle ridges of the Culbin Foreland and that an indication of their age might be had from accurately determining their height above OD. Although the area was resurveyed some two years ago the data on any new bench marks or spot heights is not yet available. ‘We are very grateful to John Tweddle of Forres for running a level from the nearest published bench mark at the top of ‘Hill 99’, the great sand dune in the centre of the forest, to the site of one of the soils at a point N.G.R. NH 992 626. Here the top of the shingle ridge was found to be at 8.43m OD. and the next ridge to the south at 9.33m. As the post glacial high sea level in this area stood at 5.2m storm ridge heights of this order are near the maximum possible throw for a beach of moderate slope so it unfortunately cannot be said with certainty that the ridges date from this period of about 5000 B.P. or from the earlier period when the sea level was falling from the late glacial high level of some 20m which occurred around 12,500 B.P.
The soil surfaces on the ridges stand at 9.20m and 10.03m respectively, with blown sand on top. Below the soil horizon, in the sand filling the troughs between the shingle ridges can be seen stratified bands of heavy minerals (mostly garnet and magnetite) and occasionally of small pebbles, indicating that this sand was laid down by water.
The soil caps on the ridges are probably all that remains of a much more extensive sheet which covered most of the shingle complex, curving in the form of a low promontary round the mouth of the River Findhorn. The gales which led to the overwhelming of the Culbin estate were it described as being so severe that the sand was blown out from below the soil. At this site it is easily demonstrated how the wind erodes the sand from the hollows between the ridges leading to the collapse and disintegration of the overlying soil, so that these historical descriptions probably pertain to this part of the foreland rather than the one-time estuarine flats which made up the slightly lower lying agricultural lands of the old estate.
At the eastern end of the shingle complex at a point NJ 028 643 other remnants of the old soil can be seen capping some small badly eroded dunes. Here the soil is a cemented podsol on top of what appears to be an old fossil dune surface Where layers showing traces of organic matter darken the sand in bands, and where there is considerable cementing by iron oxide. The very mature soil is of great thickness, showing there had been an abundant scrub flora on top, while charcoal layers in the soil are evidence of burning – probably on several occasions. The soil horizon here stands just over 3m above the shingle but due to the long distance from published bench marks the height above O.D. has not yet been determined.
As the old hooked shingle spit was moving in a Westward direction this part is older than the section described above, and if it proves to be standing on shingle at a higher level the soil may well date from the period of the falling late glacial sea.