Teesdale provides good exposures of Carboniferous Yoredale cycles on the ridge of the Caledonian Alston block. The Whin sill emplacement into the later carboniferous forms spectacular waterfalls on resistant rock. More recent glacial deposition features were also to be seen.
In Bowlees quarry a thick bedrock exposure of Scar Limestone was examined. The beds were dipping gently to the South East.
The waterfall on the Bow Lee beck seen below was flowing over the Scar limestone although the fall above that was over sandstone. The Scar limestone is visible the large block on the right of the waterfall. The slippage probably the result of movement in the fault. The stream bed showed tell tale ripple marks of sandstone. Yoredale sequences or Cyclothems are repeated geological cycles of limestone-shale-sandstone, seatearth/coal formed in the mid carboniferous.
At low force the Whin Sill predominates, its crude hexagonal columnar features stand out like broken teeth. Towards the close of Carboniferous Period some 295 million years ago, crustal stretching caused by movement of the Earth’s tectonic plates allowed the emplacement of igneous intrusions of magma across much of northern England in a suite of tholeiitic (quartz) dolerite intrusions. On cooling, these crystallised and solidified to form the Great Whin Sill seen below.
The light grey rock in the picture below looks like limestone but this is deceptive, since this is the same dolerite rock as the Sill in a weathered state.
Near Wynch bridge we looked at a sandstone raft left in the Dolerite as it solidified. The picture shows the boundary between grey/brown Whin and paler coarser grained sandstone. The point of contact is a chilled margin, where a thin dark glassy layer of tachylite can be seen.
While walking up to the High Force drumlins were conspicuous. These shallow egg shaped landforms are depositional features formed by a glacier as it retreats.
More possible evidence of glaciation seen below in the striations (scratched bedrock caused by glacial abrasion) on this rock.
High Force is an impressive waterfall where the river Tees drops 21 metres over the Whin sill into a plunge pool below.
The stratigraphy sequence can be discerned in the rock layers. The dark grey horizontal beds visible on the bottom fight are composed of the Tyne Bottom limestone. The bed directly above is sandstone and the top two vertically jointed beds are the Whin sill. The quartz dolerite magma has been injected into the sandstones and there is a chilled tachylitic margin for those intrepid enough to look for it.
Reference: Northumbrian Rocks and Landscape Ed by Colin Scrutton.