The geology of Northern England is quite complex, however there are three aspects of interest to the Field Club.The first of these is the Whin Sill. This is a quartz dolerite rock that was intruded into the limestone of Upper Teasdale as a hot magma about 295 m. years ago. In Teesdale it altered the limestone into what is known to geologists as saccharoidal marble and to biologists as sugar limestone.. Wide areas of sugar limestone soils have been produced on Cronkley and Widdybank Fells. These soils bear a relict flora which has brought botanists to these fells for many years. The Whin Sill has also had an effect on the river Tees, forming the magnificent High Force waterfall and downstream Low Force falls. Beyond our area the Whin Sill appears further north. Hadrians Wall is built on outcrops of the sill and appears again as the Farne Islands, another special place.