The following chronology and Figure 23 provides a synthesis of the geological history derived from observations
made at Tedbury Camp Quarry and across the wider Mendip region.
Figure 23. Formation of the east Mendip unconformity (after Farrant, 2008)
1. During the early part of the Carboniferous, about 340Ma, organic limestones formed in the warm, shallow seas that
teemed with life. The sediments were buried beneath a thick succession of Upper Carboniferous sandstones and
mudstones, compacted and cemented.
2. Variscan earth movements during the late Carboniferous/early Permian (290-280Ma) deformed the Mendip region into a
series of broadly east-west trending asymmetric anticlines. The limestones at Tedbury Camp form part of the steep
northern limb of one of those anticlines.
3. During the Permian and early Triassic, a period of about 50 million years, the Mendips were subject to subaerial erosion
under desert conditions. Up to 3km of Carboniferous sediment was removed from the crests of the anticlines and as
rainfall increased during the Triassic, flash-flood deposits filled the valleys excavated into the side of the denuded
mountain chain (Figure 23.1)
4. By the end of the Triassic (210Ma), the Mendip region probably formed a low-lying archipelago, surrounded by salt flats
and shallow desert playa lakes. A relative rise in sea level submerged most of the archipelago and produced an extensive
wave cut platform that terminated in a line of sea cliffs. Thin successions of shallow water sediment were intermittently
deposited and removed from the platform during small shifts in sea level throughout the early Jurassic (Figure 23.2).
5. The next major rise in sea level occurred in middle Jurassic times (170Ma) and completely submerged the area (Figure
23.3). Thousands of feet of Jurassic, Cretaceous and early Tertiary sediment was deposited before the sea retreated and
renewed erosion began some 30-40Ma. During the Tertiary, extensional movements caused minor displacements within
the residual Jurassic succession and underlying Carboniferous strata.
6. With sea levels considerably higher than at present, the Mendip area was rapidly unroofed during the later Tertiary.
Progressive falls of sea level during the Pleistocene then promoted the development of erosion platforms, particularly
under periglacial, tundra-like conditions, resulting in the ‘exhumed’ topography seen today.