Rosthwaite Fell

Also within

Cumbria County LAKE DISTRICT National Park

Condition (Natural England)

Geology & landscape

Geological Period: Ordovician

Landscape processes: Volcanic / Igneous

Description

Area, 230.63 hectares 569.88 ac Description Rosthwaite Fell lies near the south end of Borrowdale, some 11 kilometres south of Keswick. The geological interest of the site, in technical terms, may be defined as follows. The almost continuous rock exposures up the slopes of the fell reveal a wide variety of volcanic and volcani clastic rocks belonging to the Birker Fell and Airy's Bridge Formations and the Seathwaite Fell Tuffs of the Borrowdale Volcanic Group. The character of volcanicity in the middle of the Borrowdale Group may be discerned from this sequence. It illustrates the predominance of basic to intermediate lavas, in association with acidic ignimbrites, at that time. This is a historic site being the area where ancient welded ash flow tuff deposits were first recognised. In layman's terms this interest may be expressed more simply. The extensive crags and rock outcrops within this site provide outstanding exposures of a thick succession of volcanic rock layers representing a large proportion of the important episode of volcanic activity which generated the Borrowdale Volcanic Group during the middle Ordovician Period of geological history, about 460 million years ago. A wide variety of different rock types are present and well exposed within the site. This is an important site as a reference section for detailed geological study and is of historical interest as an area where certain volcanic rock types were first recognised.