Editing Geology and landscape of Upper Teesdale - an excursion

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The River Tees changes its character some 200 m upstream from the bridge, caused by the exposure of the top margin of the Whin Sill. The sill has been injected into the sandstones of the Tyne Bottom Limestone Cyclothem and now forms the bed of the River Tees for 1.7 km upstream to Holwick Head Bridge [NY 889 283]. The indurate nature of the upper margin of the Whin Sill causes the river to flow over a series of rapids, small waterfalls and through small sharply defined valleys.
 
The River Tees changes its character some 200 m upstream from the bridge, caused by the exposure of the top margin of the Whin Sill. The sill has been injected into the sandstones of the Tyne Bottom Limestone Cyclothem and now forms the bed of the River Tees for 1.7 km upstream to Holwick Head Bridge [NY 889 283]. The indurate nature of the upper margin of the Whin Sill causes the river to flow over a series of rapids, small waterfalls and through small sharply defined valleys.
  
=== Locality 8 [NY 904 278] ===
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=== Locality 8 [NY 904 278] ===
  
Here there is a large 2–2.5 m thick raft of metamorphosed sandstone in the upper part of the Whin Sill. This 74 m long section of detached roof rock has tilted to an angle of c.20° as it slowly foundered into the crystallising basic magmatic intrusion, producing glassy tachylitic chilled margins to the edge of the sill and in turn being metamorphosed.
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Here there is a large 2–2.5 m thick raft of metamorphosed sandstone in the upper part of the Whin Sill. This 74 m long section of detached roof rock has tilted to an angle of c.20° as it slowly foundered into the crystallising basic magmatic intrusion, producing glassy tachylitic chilled margins to the edge of the sill and in turn being metamorphosed.
  
=== Locality 9, Wynch Bridge [NY 903 280] ===
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=== Locality 9, Wynch Bridge [NY 903 280] ===
  
 
Cross over the nineteenth century bridge to view Low Force, a well loved river feature effected by the hard nature and crude columnar jointing of the Whin Sill. Pause on the bridge to note the full extent of the sandstone raft at Locality 8. (The walk can be terminated here. Return to the Bowlees Picnic area, noting in passing the long low, partly wooded hill to the east of the path at the start of the fields: a northwest–southeast oriented, extended, ice moulded heap of glacial debris.)
 
Cross over the nineteenth century bridge to view Low Force, a well loved river feature effected by the hard nature and crude columnar jointing of the Whin Sill. Pause on the bridge to note the full extent of the sandstone raft at Locality 8. (The walk can be terminated here. Return to the Bowlees Picnic area, noting in passing the long low, partly wooded hill to the east of the path at the start of the fields: a northwest–southeast oriented, extended, ice moulded heap of glacial debris.)
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Walking upriver on the west bank from Wynch Bridge, many features associated with the erosion of the upper contact of the Whin Sill can be noted in the river course. Also the wet nature of the associated valley alluvial deposits makes this section of Teesdale an ideal area for the study of summer wetland plants.
 
Walking upriver on the west bank from Wynch Bridge, many features associated with the erosion of the upper contact of the Whin Sill can be noted in the river course. Also the wet nature of the associated valley alluvial deposits makes this section of Teesdale an ideal area for the study of summer wetland plants.
  
=== Locality 10, Holwick Head Bridge [NY 889 283] ===
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=== Locality 10, Holwick Head Bridge [NY 889 283] ===
  
Here the nature of the Tees valley changes where the river enters a deeply incised gorge which terminates with the waterfall of High Force [NY 880 284]. Near this bridge the river crosses a major north-northwest–south-southeast fault complex which has the combined effect of some c.75 m downthrow to the east ([[:File:YGS_NORTROCK_FIG_16_2.jpg|Figure 16.2]]). Indeed the continuation of the Teesdale Fault controls the northwest–southeast direction of the first section of the High Force Gorge.
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Here the nature of the Tees valley changes where the river enters a deeply incised gorge which terminates with the waterfall of High Force [NY 880 284]. Near this bridge the river crosses a major north-northwest–south-southeast fault complex which has the combined effect of some c.75 m downthrow to the east ([[:File:YGS_NORTROCK_FIG_16_2.jpg|Figure 16.2]]). Indeed the continuation of the Teesdale Fault controls the northwest–southeast direction of the first section of the High Force Gorge.
  
This locality may also be approached from the public car park and picnic site at High Force [NY 886 287].
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This locality may also be approached from the public car park and picnic site at High Force [NY 886 287].
  
 
=== Locality 11, High Force [NY 881 284] ===
 
=== Locality 11, High Force [NY 881 284] ===

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