Geoscience Reference
In-Depth Information
ish and salt water (shallow sea and coastal). The area was sinking during Devoni-
an times, allowing sediment to accumulate along a broad swathe as the coastline
moved progressively inland. During the convergent movements that caused the Var-
iscan mountain building, the Devonian sediments were moved into folds and sliced by
fractures, and the mudstones were cleaved to form slates.
The next bedrock episode represented is the New Red Sandstone. This was de-
posited after the Variscan mountains had grown as a result of the plate convergence
and compression of the crust. The New Red Sandstone was deposited on and against
the eroded surface of the deformed older material, as outlined in Chapter 4, and was
itself the product of erosion of the higher areas of the Variscan mountains. The New
Red Sandstone consists of distinct layers of sandstone, conglomerate and mudstone,
and there is an overall trend towards finer-grained material in the younger layers (for
example, the very fine-grained Mercia Mudstones of Triassic age). This finer material
indicates a trend towards less vigorous river transport of debris as the high ground of
the Variscan mountains was being carved and lowered by erosion, leading to gentler
river channel gradients. Much of the New Red Sandstone was deposited by rivers and
in lakes on land, though in later times the area was sometimes submerged by the sea.
Lying on top of the New Red Sandstone, the Jurassic bedrock succession in the
west outcrops most dramatically along the
Jurassic Coast
, a World Heritage Site con-
sidered in more detail in Area 4. For now, it is enough to note that the full succession
represents most of the 50 million years of Jurassic time, and consists mainly of mud-
stones, thin limestones and some sandstones that accumulated in the seas of the South-
east England Basin. Many types of fossils have been found in these rocks, and in some
localities they are extremely abundant. The Jurassic ammonites are particularly fam-
ous, and form the basis for dating this succession, while fossil vertebrates - including
dinosaurs - have played an important role in the early days of geological science and
are still being studied today. The layers in the Jurassic that have resulted in extensive
topographic features are primarily the Bridport Sand (from the Early Jurassic) and the
limestones, especially the Purbeck and Portland limestones from the very latest Juras-
sic.
