Editing Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion

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Turn north at the Milecastle Inn [NY 716 660] to Cawfields. Immediately on the right is a good section of the Four Fathom Limestone exposed in an old quarry, while on a raised hillock on the left, between the road and Haltwhistle Burn, is the site of the fortlet that guarded this section of the Stanegate. Between the Four Fathom Limestone and the top surface of the Whin are the remains of old workings in coal below the Three Yard Limestone and '''siderite''' ironstone bands in shales in the same part of the succession. The most striking feature, however, is the vallum and mounds running along the base of the Whin feature. Continue to Cawfields quarry car park.
 
Turn north at the Milecastle Inn [NY 716 660] to Cawfields. Immediately on the right is a good section of the Four Fathom Limestone exposed in an old quarry, while on a raised hillock on the left, between the road and Haltwhistle Burn, is the site of the fortlet that guarded this section of the Stanegate. Between the Four Fathom Limestone and the top surface of the Whin are the remains of old workings in coal below the Three Yard Limestone and '''siderite''' ironstone bands in shales in the same part of the succession. The most striking feature, however, is the vallum and mounds running along the base of the Whin feature. Continue to Cawfields quarry car park.
  
==== Locality 3, Cawfields Quarry [NY 713 666] ====
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==== Locality 3, Cawfields Quarry [NY 713 666] ====
  
The car park and lake are on the site of the old whinstone quarry which removed not only the scarp of the Sill but also the Roman Wall that ran along its top; working ceased when they approached Milecastle 42 and the break in the continuity of the escarpment.
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The car park and lake are on the site of the old whinstone quarry which removed not only the scarp of the Sill but also the Roman Wall that ran along its top; working ceased when they approached Milecastle 42 and the break in the continuity of the escarpment.
  
The top surface of the Sill defines the land surface; a thin skin of '''metamorphosed''' sediment can be found in places. Columnar jointing is developed perpendicular to the top, cooling surface, of the intrusion. In the old quarry walls on the south side of the car park the fine grained chilled margin of the sill can be seen, but is very thin. More obvious is the band of vesicles 2 m below the top. These formed by gas, released from the magma by reduced pressure as it rose towards the surface. Their presence at a definite level in the sill suggests that the magma close to the surface contact had cooled to a viscosity that prevented the bubbles rising any further, most of the vesicles have later been filled with '''calcite'''.
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The top surface of the Sill defines the land surface; a thin skin of '''metamorphosed''' sediment can be found in places. Columnar jointing is developed perpendicular to the top, cooling surface, of the intrusion. In the old quarry walls on the south side of the car park the fine grained chilled margin of the sill can be seen, but is very thin. More obvious is the band of vesicles 2 m below the top. These formed by gas, released from the magma by reduced pressure as it rose towards the surface. Their presence at a definite level in the sill suggests that the magma close to the surface contact had cooled to a viscosity that prevented the bubbles rising any further, most of the vesicles have later been filled with '''calcite'''.
  
Along the north side of the lake, a stile over the low stone wall permits access to the main quarry face, which at its northern end exposes the base of the sill, resting on sandstone dipping southwest at 45°. To the east, behind the quarry face, the Whin escarpment is offset along the line of a small northwest–southeast valley. The valley may mark the position of a fault, '''downthrowing''' east, displacing the Whin escarpment to the north. Recent geophysical work, however, suggests that the displacement is due to a transgression of the sill and it seems probable that both faulting and transgression are involved. On the east side of the small valley Milecastle 42 occupies a sloping site and has short stubs of broad wall on either side joining on to the narrower linking wall section.
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Along the north side of the lake, a stile over the low stone wall permits access to the main quarry face, which at its northern end exposes the base of the sill, resting on sandstone dipping southwest at 45°. To the east, behind the quarry face, the Whin escarpment is offset along the line of a small northwest–southeast valley. The valley may mark the position of a fault, '''downthrowing''' east, displacing the Whin escarpment to the north. Recent geophysical work, however, suggests that the displacement is due to a transgression of the sill and it seems probable that both faulting and transgression are involved. On the east side of the small valley Milecastle 42 occupies a sloping site and has short stubs of broad wall on either side joining on to the narrower linking wall section.
  
 
If time permits, the walk westwards from Cawfields via the fort of ''Aesica ''to Walltown, largely on the Whin dip slope, is very rewarding archaeologically.
 
If time permits, the walk westwards from Cawfields via the fort of ''Aesica ''to Walltown, largely on the Whin dip slope, is very rewarding archaeologically.

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