The Peninsula Emerges - Gower - page 29

from the south and layers ofcarbonates gradually built upunder a very shallow sea that was rich in corals
and brachiopods, sedentary shelled animals with feathery appendages to waft food particles to the mouth.
The uppermost part of the Carboniferous sequence in Gower consists of marine shales and the muddy
limestones of the Oystermouth Beds. These contain the widespread limestone fauna of brachiopods and
corals, together with the rare trilobite Griffithsides spp. Brachiopods are the most abundant fossils, espe-
cially Martinia spp. and Spirifer spp. One of the Spirifer species is named Spirifer oystermouthensis after
Oystermouth village, where it is foundin Clements Quarry,the so-called 'Black Lias Quarry', named due
to the alternations of dark, muddy limestones (which weather to white) with thin dark, calcareous shales.
There are many theories about howthis regular pattern was produced, but there is a strong possibility that
ithadsomeseasonalorclimaticcause.Alsopresentinthequarry,whichiseasytoaccessasitiscurrently
used as a car park, is the small cornet-shaped shell Zaphentis oystermouthensis .
From time to time the carbonates were above sea level and there is evidence of erosion and of plant
roots. Unusual breccias (rocks composed of angular fragments) suggest the presence of salts formed by
the evaporation of sea water in a hot climate. Other deposits suggest lagoons and oolite, a limestone com-
posedofsmallgrains,comparablewiththoseformingtodayinshoalsaroundtheBahamas.Insomelevels
there are algal tufts and mats that formed as calcareous layers built up by lime-secreting algae that fa-
voured shallow, warm seas. These varying conditions produced different types of limestone, and six main
rock units can be identified, all named after areas on the peninsula (Table 2).
During the Carboniferous period forces that were eventually to fold and fault the Gower rocks were
beginning to bend the earth's crust upwards. This increased the runoff from the land, which in turn resul-
tedinriverdeltasswampingthelimestone seawithsediment. IncomparisontootherareasofGlamorgan,
where the rocks show that there were alternating periods of non-marine and marine conditions, the sea
over Gower at this time, some 320 million years ago, was comparatively deep, and fine shales were de-
posited with marine animals preserved within them. This fine material, known as Namurian Shales, is re-
latively softandwassubsequently erodedtoformthebaysofOystermouth, OxwichandPort-Eynon.The
best section through the Namurian rocks occurs along Barland Common Stream, near Bishopston, where
the rock sequence contains a number of bands with marine fossils. The section is of historical interest as
it was originally recorded by de la Beche and described in the first Geological Survey Memoir published
in 1846.
TABLE 2. Simplified stratigraphy of Gower limestone. (Adapted from Lowe, 1989)