Swithland Wood - Geology

Geology

The ancient rocks that characterise Charnwood Forest are an eroded anticline - the layers of sediment built up on a sea floor were uplifted some 420 million years ago, at the end of the Silurian period. This created a dome, the top of which was eroded to expose successively more ancient rocks. The oldest rocks are found at the northern core, at Blackbrook Reservoir, while Swithland Wood lies on the south-eastern edge of the anticline. The Swithland Slates and other rocks of the 'Brand Group' are the most recent of the Charnian rocks. For most of the 20th century they had been classified as Precambrian rocks along with the more central Charnwood outcrops. This would have dated them to around 545 million years ago. However, recent discoveries of trace fossils, evidence of animals burrowing in the soft mud that became the Swithland Slate, have reclassed all of the Brand Group rocks as Cambrian, formed around 530 million years ago. Although the Charnwood Precambrian rocks have the internationally significant fossils of the frond-like lifeforms of Charnia species, clear evidence of trace fossils of burrowing animal life places the Swithland Formation firmly in the Cambrian era. Some of these trace fossils can be seen on Swithland Slate headstones, such as those in Ratby churchyard.

The mud that became Swithland Slate was deposited in great quantity on the sea bed, after the more dramatic Precambrian volcanic activity had subsided. It was very fine sediment resulting in fine-grained evenly bedded material. Subsequent deposits above it, combined with the uplifting of the anticline, produced the heat and pressure which turned the mud into hard rock (Lithification). The tensions created during uplift are what created the cleavage plane in the slates, and it is along these cleavage joints (rather than the bedding planes) that the slates are split to create roof slates. With the Charnwood area uplifted into a high mountain range, the processes of erosion began.

By the Triassic period, (240 million years ago) the mountains were eroded down to something like their current height. There then began a new deposition, this time of desert sand and fine dust, that produced a new layer of soft red marl, creating the gentler rolling appearance of the land, with just the tips of the older rocks reaching the surface. Over most of Swithland Wood the marl is the surface rock type (although the slate beds will occur underneath that).

Finally, during the ice age, glacial erosion stripped back the Triassic material (plus whatever had accumulated above that), to re-expose the eroded peaks of the older rocks. At the end of the ice age, patches of boulder clay were deposited over much of the local area, including parts of Swithland Wood.