Editing Dalradian rocks of Jura – an excursion

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== Introduction ==
 
== Introduction ==
[[File:DalradianExcursion012.jpg|thumb|General map of the area described in this guide. The Ordnance Survey 1:50,000 sheet 61 covers the whole of Jura.]]
 
 
'''General description of guide '''
 
'''General description of guide '''
 
The Middle Dalradian rocks of Jura are of particular interest because, as a result of their mild deformation and metamorphism (lowest greenschist facies), a large proportion clearly exhibit original sedimentary structures. Although deformational structures and minor intrusions will be referred to, the main purpose of the excursions is to demonstrate the sedimentological features of a sequence of various marine facies. For the sake of simplicity the rocks will often be referred to as muds and sands although they are now mainly slates, phyllites and very hard quartzites. Exposure is good throughout the area, especially in the wave-cut benches of raised beaches from which many of the localities are described.
 
The Middle Dalradian rocks of Jura are of particular interest because, as a result of their mild deformation and metamorphism (lowest greenschist facies), a large proportion clearly exhibit original sedimentary structures. Although deformational structures and minor intrusions will be referred to, the main purpose of the excursions is to demonstrate the sedimentological features of a sequence of various marine facies. For the sake of simplicity the rocks will often be referred to as muds and sands although they are now mainly slates, phyllites and very hard quartzites. Exposure is good throughout the area, especially in the wave-cut benches of raised beaches from which many of the localities are described.
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=== General geology ===
 
=== General geology ===
[[File:DalradianExcursion013.jpg|thumb|Stratigraphical relationships of the formations described in this guide.]]
 
 
There are no really useful publications which describe the geology of the area. Most of the early work was stratigraphical and Bailey's (1916) summary and map are still the most authoritative. The Geological Survey memoirs (Peach et at 1909, 1911) include much lithological detail and descriptions of the Caledonian and post-Caledonian dykes and sills. anderton (1976) has recently discussed the depositional environments of the Jura Quartzite.The stratigraphical relationships of the formations are shown in the Figure. The Jura Quartzite is interpreted as having been deposited on a tidal-current dominated shallow marine shelf. As one follows the predominant palaeocurrent direction towards the NNE. the sediments change from almost entirely cross-bedded sands, deposited from migrating mega-ripples or sand-waves, to include an increasing proportion of laminated and rippled sands and silts. This is very similar to the change observed down the bedload sediment transport path in the present day Celtic Sea (Belderson and Stride 1966). There is no obvious relationship between such tidal palaeocurrents and the shoreline, but they may be very approximately parallel.
 
There are no really useful publications which describe the geology of the area. Most of the early work was stratigraphical and Bailey's (1916) summary and map are still the most authoritative. The Geological Survey memoirs (Peach et at 1909, 1911) include much lithological detail and descriptions of the Caledonian and post-Caledonian dykes and sills. anderton (1976) has recently discussed the depositional environments of the Jura Quartzite.The stratigraphical relationships of the formations are shown in the Figure. The Jura Quartzite is interpreted as having been deposited on a tidal-current dominated shallow marine shelf. As one follows the predominant palaeocurrent direction towards the NNE. the sediments change from almost entirely cross-bedded sands, deposited from migrating mega-ripples or sand-waves, to include an increasing proportion of laminated and rippled sands and silts. This is very similar to the change observed down the bedload sediment transport path in the present day Celtic Sea (Belderson and Stride 1966). There is no obvious relationship between such tidal palaeocurrents and the shoreline, but they may be very approximately parallel.
  
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=== Locality 4: Glentrosdale Bay  ===
 
=== Locality 4: Glentrosdale Bay  ===
[[File:DalradianExcursion015.jpg|thumb|General map of the area described in localities 4,5 & 6.]]
 
 
Sedimentary structures in the Jura Quartzite.
 
Sedimentary structures in the Jura Quartzite.
  

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