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It is proposed that the shrinkage of well-decomposed peat beneath conifer plantations owing to the irreversible loss of water takes place in two stages. The first stage of pure subsidence is followed by a second stage of the formation of natural aggregates with a network of large cracks and a slower rate of subsidence. A plot of specific volume against gravimetric water content for natural aggregates or for excised blocks dried artificially shows a linear relationship throughout the range of water content. The assumption of equi-dimensional shrinkage in stage two allows a prediction of the relative amounts of subsidence and crack formation. The shrinkage of non-cracking fibrous peat is also dealt with. A preliminary attempt at verification of the predictive model from measurements of crack volume or subsidence of ground level shows that the model gives values of the right order of magnitude. I N T R O D U C T I O NThe extensive blanket peatlands of upland Britain (and Ireland) are characterized by peat with a high degree of decomposition and a remarkable ability to resist rewetting once partially dried, a property that has for centuries enabled it to be prepared for use as a domestic fuel, simply by exposing blocks of peat to the atmosphere for several weeks even in the cool rainy weather experienced in these regions.The process of ripening of the clayey sediments freshly exposed on the Dutch polders, following drainage and the growth of pioneer vegetation, involves the irreversible loss of water, subsidence of the surface and the development of shrinkage cracks in the upper layers (Rijniersce, 1982). Volume changes of a similar nature occur in the blanket peatlands following afforestation by conifers (Binns, 1968; Burke, 1978; Pyatt, 1976 Pyatt, , 1987. This paper suggests a model by means ofwhich such volume changes may be analysed in relation to the peat water-content.Blanket peat usually comprises three layers of which the middle one is the most variable in thickness. The upper layer or acrotelm (Ingram, 1979) of 10-40 cm is least decomposed (Von Post humification (H) value 2-5) and consists of strong fibrous material and live roots within a dark brown or black humified matrix. Over a depth of 1&20 cm this gives way to pseudofibrous peat (Fraser, 1933) in which fibres are quite distinct but have lost much of their strength. When squeezed, the peat mass is extruded between the fingers with little or no release ofwater (H 7-9). Pseudofibrous peat is initially reddish brown but darkens within minutes of being exposed to oxygen. The lowest blanket peat layer is amorphous, 5&100cm thick, dark brown or black and denser than the overlying material. The amorphous layer usually contains ancient stumps of pine or birch and merges with the organic-rich layer of mineral soil beneath. The layers of pseudofibrous and amorphous peat constitute the catotelm (Ingram, 1979). In shallow blanket peats (less than 1.0 m depth) the pseudofibrous layer is usually missing. These are the sites which are favoured for cutting domesti...