The rocks of Stybarrow Dodd are all part of the
Borrowdale Volcanic Group (BVG), 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 and its ridges belong to the
Birker Fell Andesite Formation. These rocks are among the earliest of the volcanic rocks of the BVG, 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. 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
basaltic andesite occurring in a number of places. Individual lava flows may be separated by beds of
volcaniclastic sandstone, sedimentary deposits formed from the erosion of the volcanic rocks. The geological map shows basaltic andesite near Fisher Place, low on the west ridge of Stybarrow Dodd, as well as small amounts of volcaniclastic deposits on the same ridge. After the eruptions of the Birker Fell Formation the composition of the erupting magma changed from andesitic to
dacitic, and as a result the nature of the volcanism became more explosive. A large
caldera volcano was built in the area where Scafell and the Langdale Pikes stand today. The caldera was at least across, and as the caldera collapsed, much more than of magma were erupted. At the close of this volcanic activity, the caldera was filled with sedimentary deposits, represented today by the
Seathwaite Fell Sandstone Formation, a sequence of bedded volcaniclastic sandstone and siltstone strata. However, deposition was not confined to the old caldera, and to the east of Thirlmere this formation rests directly on the Birker Fell andesites. On the west ridge of Stybarrow Dodd, the geological map shows a thin deposit of the Seathwaite Fell Sandstone around the 700 m contour. Another caldera volcano then formed in a new area. In the area to the north of Sticks Pass, the Birker Fell andesites (and the small area of Seathwaite Fell Sandstone) 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 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 Stybarrow 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. ==Names==