No description available.

No description available.

No description available.

No description available.

The Upper Lias of Yorkshire as represented to the north and west of the Peak Fault, is a phase of aeposition of argillaceous sediments under varying physical conditions. The Grey Shales are shallow water deposits which pass gradually but rapidly into the finely laminated, bituminous shales of the Jet Rock, which were laid down under 'Black Sea conditions', but in water much shallower than the Black Sea. As the basin was gradually filled up during Bituminous Shale and Hard Shale times, its floor rose into the zone of aerated water, when the poorly bedded Alum Shales were deposited. Thes sedimentational conditions were interrupted by the deposition of an argillaceous limestone at the end of Jet Rock-times and by several beds and rows of masses of siderite mudstone, at varying horizons during subsequent zone-moments. The best 'hard' jet is found only in the Jet Rock, and is entirely composed of collapsed and compressed wood structures, soaked in humic substances resulting from the decomposition of the original wood. Annual rings, medullary rays, tracheids, bordered pits, stem bases and bark are all recognised in the jet. Some specimens of jet are silicified along a central zone, and here the wood structure is well preserved in an uncrushed state. Within the jet are frequent rows of included quartz grains and other minerals, which had become wedged into cracks in the wood before sedimentation. Whitby jet is the altered ermains of coniferous wood washed from the Liassic land surface into the Yorkshire Basin, where it became waterlogged and sank. The dominating factors in the subsequent jetonisation were the conditions of reduction and stagnation in the deeper parts of the basin during Jet Rock times. These conditions caused the formation of a fine, black viscous mud which, together with the absence of oxygen due to the same reason, controlled the decomposition of the wood and limited it to a unique and prolonged breakdown of organic constituents. As a result the wood which was in the form of stems and trunks, was reduced to a pulpy condition. The constant slow deposition of muds caused the stems to be much flattened and to assume the form in which they are now found. The inferior varieties of jet are due to formation in more oxygenated conditions of sedimentation, and probably to a greater degree of aerobic decomposition before incorporation in the argillaceous sediments. The increased amount of oxygen would cause decomposition to follow a more normal course.

The mid to Late Holocene environmental development of the Sedgemoor Valley, Somerset Levels is examined using a multidisciplinary approach involving lithostratigraphy, biostratigraphy and radiocarbon dating. Three sites along a transect of the valley and one in the Bristol Channel are studied producing a temporal and spatial model of its palaeoenvironmental development. A common red clay is seen in all three sites which has been interpreted as a palaeosol and its surface varies between -1 .33 m 00 and 6.2 m 00 mantling the bedrock topography. Lithostratigraphic evidence indicates the red palaeosol was inundated by mid Holocene sea level rise as it is overlain by a blue clay that contains marine foraminifera. The surface of this blue marine clay varies in altitude between -1 . 1 4 m 00 and 5.51 m 00, above which a peat deposit occurs demonstrating a marine regression and a change to freshwater conditions. Radiocarbon analysis has been carried out on five peat samples immediately above the clay-peat contact to date the marine regression. They vary from between 6920 and 6450 cal. yrs BP at a altitude of -0.87 m 00 and 4795 and 4 1 70 cal . yrs.BP at 4.71 m 00. This study supports previous research suggesting that the peat began to form at a low altitude and subsequently extended vertically and laterally to cover the exhumed clay surface. The peat deposit initially contains Phragmities and then becomes either detrital or turfa in nature up core. The detrital peat contains freshwater molluscs indicating that large lake-like fringing waterbodies persisted in the valley for much of the late Holocene. Later the influence of humans is seen in the lithostratigraphy as drainage works become evident, marked by a change from organic to more minerogenic deposition. A saltmarsh at Stert is studied to examine the current intertidal area, and to calibrate and partially fill the gap in the stratigraphical record between the end of the peat samples in the valley and today.

The problem is dealt with under two main headings, a) the short- term and b) the long-term effects of burning. Short-term effects. 1. On the vegetation. Calluna and the dwarf shrubs may be completely destroyed by fire, but species that are caespitose or have protected underground parts commonly survive. Regeneration of the dwarf shrubs is facilitated by their high seed-production, but they can regenerate vegetatively if not killed. The interval between successive burnings ('cycle-length'), and not fire damage, appears to be a major factor in determining the floristic composition of the vegetation. 2. On the soil. The base-status of the upper soil horizons declines through each cycle. Leaching experiments show that an increased amount of base is lost in the run-off water and leachate soon after burning. It is concluded, in view of the restriction of the rooting systems to the upper soil, that there is in this way an appreciable loss of bases from the peaty horizons at each burning. Long-term effects. 1. On the vegetation. There is evidence that 150 years ago heather moors were considerably richer in species than they are today. It is also shown that long-continued systematic burning leads to a greater loss of species than irregular and less frequent burning.2. On the soil. In all cases examined, woodland soils show a higher base-status than closely comparable soils under moorland, and it seems clear that the fertility of the moors is lower than it would have been had their former woodland persisted. The phytometrical use of green heather leaves substantiates these findings, and shows the fertility of old moors to be less than that of moors of recent origin. In the final section the economic utilisation of heather moor is discussed.

No description available.

No description available.

No description available.