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 Period. Some of these trace fossils can be seen on Swithland Slate headstones, such as those in Ratby churchyard.