Upper Teesdale is a special place. Vast upland fells, bleak in places, give way to rough pastures and the meadows on the banks of the River Tees. Geology, glaciation and climate have created a unique habitat that makes Upper Teesdale world-renowned as a botanical hotspot.
Stepped valley side above Holwick Scar: M Rogers
The Yoredales Series of sedimentary rocks, a repeating sequence of limestone, shale, sandstone and occasionally thin coal seams, were formed about 300 million years ago in the Carboniferous Period in tropical seas and river deltas. These alternating bands of hard and soft rocks form the flat-topped fells and stepped valley sides on the Pennine Dales.
The Whin Sill
About 295 million years ago, hot molten lava from deep in the earth penetrated the rocks of the Yoredales Series, filling cracks and forcing its way into weaknesses between the sedimentary layers. This formed huge sheets that we know as the Whin Sill, a very hard doloritic rock which forms many of the larger cliffs and waterfalls of Teesdale.
The waterfall at High Force flows over the Whin Sill, with limestone and sandstone from the Yoredales Series forming the lower part of the falls: MRogers
Where the hot lava which formed the Whin Sill came in contact with the limestones of the Yoredales Series, it sometimes baked these to form a type of marble called the Sugar Limestone. The weathered surface of Sugar Limestone is home of many of the rarest Teesdale flowers.