The Cumbrian part of Teesdale was historically divided between Cumberland and Westmorland, and the County Durham area between the former and Yorkshire.

Parts of the local climate have been scientifically classified as "Sub-Arctic", and snow has sometimes lain on Cross Fell into June.

[2][3] Unusually for the Pennines, rock of igneous origin (the Whin Sill) contributes to the surface geology and scenery of Upper Teesdale.

Around 295 million years ago upwelling magma spread through fissures and between strata in the earlier Carboniferous Limestone country rock.

In places this impervious dolerite rock, with shallow soil above it, prevented the growth of scrub or trees: this enabled certain post-glacial Arctic / Alpine plants to survive here when elsewhere as a rule they were overgrown.

[7] Part of Upper Teesdale near the Cow Green Reservoir is designated a National Nature Reserve; it contains the unique Teesdale Violet and the blue Spring Gentian as well as more common Pennine flowers such as rockrose, spring sandwort, mountain pansy, bird's-eye primrose and butterwort.