Anthropogenic sedimentation in Orkney : the formation of deep top soils and farm mounds
The formation of two sediment types in Orkney is elucidated. Both are demonstrated to be the result of anthropogenic sedimentary processes, deep top soils the result of arable activity, farm mounds the result of habitation activity. Deep top soils commenced formation c 1200 AD either as a spontaneous innovation due to increasing population pressure or as a new agricultural technique introduced with monastic settlement. Cessation of deep top soil formation is attributed to the 1800s agricultural improvements when new forms of land fertilizer were introduced. The land use associated with deep top soil formation was the tunmal, the most intensively cultivated part of the early township. The materials used to sustain this intensive cultivation resulted in deep top soil formation. These materials were dominantly turf from the hill land and grazing land together with variable quantities of animal manure and a little seaweed. Deep top soils are located in West Mainland on naturally less fertile soils, where seaweed was in short supply and where population density was relatively high. Farm mound formation commenced over a thousand year period, between the Iron Age and late Norse period. The major impetus to farm mound formation was the Norse settlement commencing c 800 AD. Two farm mounds examined in detail indicate a mound core was deposited using primarily a turf and manure mixture. At one site this alleviated a flooding hazard. The core was then covered with turves and peat, creating a living surface upon which pathways and fertilized garden plots are evident. In the latter stages of farm mound formation their use was as a midden where toft wastes, dominantly ash, were deposited. Farm mounds are restrictedin their distribution to Sanday and North Ronaldsay where early population levels were greatest and where ample seaweed was available for land fertilization instead of toft wastes.