Editing Dundrennan Ranges, Gipsy Point - an excursion

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==== 3 Gipsy Point: sand volcanoes ====
 
==== 3 Gipsy Point: sand volcanoes ====
[[File:P258913.jpg|thumb|300px|left|Gipsy Point. Sand volcano. P258913]]Directly overlying the large slump is a single layer (now vertical) of spectacular sand volcanoes, originally described by Lovell (1974), who suggested that they formed as a result of dewatering of the underlying slump. Above lies a coarsening-upward sequence of thin sandstone–mudstone layers, interpreted as an interchannel or overbank sequence. The sharp junction between the top of the slump, with its attendant sand volcanoes, and the overlying beds is very well seen in the cliff face. This shows clearly that the original channel had been abandoned by the time renewed deposition began.
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Directly overlying the large slump is a single layer (now vertical) of spectacular sand volcanoes, originally described by Lovell (1974), who suggested that they formed as a result of dewatering of the underlying slump. Above lies a coarsening-upward sequence of thin sandstone–mudstone layers, interpreted as an interchannel or overbank sequence. The sharp junction between the top of the slump, with its attendant sand volcanoes, and the overlying beds is very well seen in the cliff face. This shows clearly that the original channel had been abandoned by the time renewed deposition began.
  
 
Lovell identified ten sand volcanoes, occurring ill three groups, over 100 m of strike section. Five of these are easily accessible, even at mid-tide. The largest and most obvious is over 3 m across, and is almost completely preserved. It is about 1.5 m deep and has a depressed central crater nearly 0.5 m across. Several of the other volcanoes are cut across, showing laminated sandstone with complex internal structure, as if the water had escaped in a series of pulses. One small volcano directly overlies another smaller one. A thin sandstone bed connects the sand volcanoes of the landward and central groups, but does not extend to the seaward set. In this latter group, however, is a sand volcano, cut so as to display the central pipe, which in this case is filled with coarse sandstone. Stringers of gritty material have developed in the underlying sandstone, and penetrated upwards into the sediment overlying the volcano during the dewatering process. This example is the only one to show a direct connection between the beds above and below the volcano.
 
Lovell identified ten sand volcanoes, occurring ill three groups, over 100 m of strike section. Five of these are easily accessible, even at mid-tide. The largest and most obvious is over 3 m across, and is almost completely preserved. It is about 1.5 m deep and has a depressed central crater nearly 0.5 m across. Several of the other volcanoes are cut across, showing laminated sandstone with complex internal structure, as if the water had escaped in a series of pulses. One small volcano directly overlies another smaller one. A thin sandstone bed connects the sand volcanoes of the landward and central groups, but does not extend to the seaward set. In this latter group, however, is a sand volcano, cut so as to display the central pipe, which in this case is filled with coarse sandstone. Stringers of gritty material have developed in the underlying sandstone, and penetrated upwards into the sediment overlying the volcano during the dewatering process. This example is the only one to show a direct connection between the beds above and below the volcano.
  
[[File:P259111.jpg|thumb|300px|right|Gipsy Point. Sand volcano, (0986/29, rack 2). P259111]]The thin sandstone beds of the overbank sequence directly overlying the sand-volcano horizon show some interesting sedimentary structures. Firstly there are ripple marks with slightly different orientations, seen on successive vertical faces near the cliff edge. Secondly there are other structures, well worth examining, on the lower surfaces of the thin sandstone units down on the shore, a few metres above the sand volcanoes. Some of these faces are smooth but show convolute lamination in cross-section; other surfaces display very fine load casts. In some instances an otherwise smooth surface has only one or two isolated load casts, of which some are simple, whereas others are grouped together like bunches of grapes. Many faces, however, are entirely covered with load casts. One surface, about a metre above the sand volcanoes, displays strings of clearly orientated load casts, suggestive of modified ripples.
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The thin sandstone beds of the overbank sequence directly overlying the sand-volcano horizon show some interesting sedimentary structures. Firstly there are ripple marks with slightly different orientations, seen on successive vertical faces near the cliff edge. Secondly there are other structures, well worth examining, on the lower surfaces of the thin sandstone units down on the shore, a few metres above the sand volcanoes. Some of these faces are smooth but show convolute lamination in cross-section; other surfaces display very fine load casts. In some instances an otherwise smooth surface has only one or two isolated load casts, of which some are simple, whereas others are grouped together like bunches of grapes. Many faces, however, are entirely covered with load casts. One surface, about a metre above the sand volcanoes, displays strings of clearly orientated load casts, suggestive of modified ripples.
  
 
Small dolomitised concretions can be seen within the slump close to the base of the cliff, and several small fault-planes within the vicinity are likewise dolomitised. These may possibly have been synsedimentary faults penetrated by the diagenetic fluids associated with the concretion-forming process.
 
Small dolomitised concretions can be seen within the slump close to the base of the cliff, and several small fault-planes within the vicinity are likewise dolomitised. These may possibly have been synsedimentary faults penetrated by the diagenetic fluids associated with the concretion-forming process.

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