page_title: Great Dodd
section_title: Geology
hierarchical_section_title: Great Dodd / Geology
caption_attribution_description: English: Welded ignimbrite of the Thirlmere Tuff Member (Lincomb Tarns Tuff Formation) - found on the path between Great Dodd and Stybarrow Dodd
context_page_description: Great Dodd is a mountain or fell in the English Lake District. It stands on the main ridge of the Helvellyn range, a line of mountains which runs in a north-south direction between the lakes of Thirlmere and Ullswater in the east of the Lake District. Great Dodd, with a height of 857 m is the highest of the fells in this range to the north of Sticks Pass.
  Walkers may approach Great Dodd from either High Row near Dockray to the east, or from Legburthwaite to the west – or along the main ridge track from either north or south. Scramblers with climbing skills may be attracted to three gill climbs on the western side of the mountain.
  The summit of Great Dodd is a smooth, grassy, rounded dome, like its two southern neighbours, Watson's Dodd and Stybarrow Dodd. Together, these three are sometimes called ‘The Three Dodds’. These three are made of volcanic rocks of the Borrowdale Volcanic Group, and the tops of all three are covered by the same sheet of rock, which was formed in a series of huge volcanic explosions accompanying the formation of a volcanic caldera about 450 million years ago.
context_section_description: The rocks of Great Dodd are all part of the Borrowdale Volcanic Group, formed on the margin of an ancient continent during a period of intense volcanic activity, roughly 450 million years ago in the Ordovician Period.
  Within that Group, the bulk of the rocks forming the fell belong to the Birker Fell Andesite Formation. These rocks are among the earliest of the volcanic rocks of the Borrowdale Volcanic Group, and are part of a thick succession of andesite sheets which now outcrop in a wide band around the western and northern sides of the Lake District, and probably underlie much of the other volcanic rocks. These sheets were formed by successive eruptions of mobile andesitic lava from shallow-sided volcanoes. The composition of the erupting magma varied from time to time, with domes of more viscous dacite occurring in a number of places. Calfhow Pike, Randerside and High Brow are all shown as dacite on the geological map.
  Individual lava flows may be separated by beds of volcaniclastic sandstone, sedimentary deposits formed from the erosion of the volcanic rocks and deposited by streams, floods and possibly wind. The geological map shows a number of areas of volcaniclastic sandstone on the western slopes of Great Dodd, above Beckthorns and Fornside.
  After the eruptions of the Birker Fell Formation, the composition of the erupting magma changed from largely andesitic to predominantly dacitic, and as a result the nature of the volcanism became more explosive.
  A number of caldera volcanoes were formed. In the area to the north of Sticks Pass the Birker Fell andesites are overlain by the Lincomb Tarns Tuff Formation. This formation is one of the most widespread of the volcanic rocks of the Lake District; it seems that the whole district was buried beneath at least 150 m of densely welded ignimbrite, a rock formed from a pyroclastic flow of very hot gas and rock. This formation must represent a series of eruptions of truly exceptional magnitude, accompanying the formation of a volcanic caldera probably in the area around what is now Helvellyn.
  On Great Dodd this formation is represented by rocks of the Thirlmere Tuff Member, which covers the top of the fell with a thick sheet of welded rhyo-dacitic lapilli-tuff in which the individual pieces of semi-molten lava were flattened under the weight of deposits above them. This rock weathers to a white or pink colour, but it is covered by the smooth grassy turf characteristic of Great Dodd, Watson's Dodd and Stybarrow Dodd, all of which are covered by the same sheet.
  Towards the end of, or after the cessation of the volcanic activity, a large granite batholith was emplaced beneath the volcanic rocks of the Lake District. Associated with the granite batholith was the creation of numerous mineral veins.
  There is evidence of historic mining activity in two locations on Great Dodd in efforts to exploit such mineral veins. Fornside Mine on the western slopes consisted of two levels, one of which shows signs of copper ore, the other was on a barren quartz vein. At the base of Wolf Crags there are the remains of a short level 30 m long on a poor quartz vein. There are no historical records of this level. It may be assumed that both mines were commercial failures.
caption: A piece of welded ignimbrite of the Thirlmere Tuff Member, showing flattened lapilli — found on the path between Great Dodd and Stybarrow Dodd
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