Geology of the Parish

The Geology of the Parish


Thurstaston and Irby are underlain by red sandstone formed in the Triassic era (251 to 199 million years ago). As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic.  Both the start and end of the Triassic were marked by major extinctions.  The start of the Triassic period (the 'Lower Triassic') came at the end of the Permian period when up to 96 percent of all marine species and 70 percent of land vertebrate species died out.

The Triassic climate was probably hot and dry.  Land was formed together in one mass called by geologists 'Pangea'.  Pangea was surrounded by ocean except that one part of the land was deeply indented by a sea called the Tethis Ocean.  There does not seem to have been any glacial activity and the whole environment was ideal for the formation of red bed sandstones and mudstones in the ocean and by the action of wind forming sand dunes which later hardened into stone, on land.  Because of the mass extinction at the start of the Triassic period there are few fossils in the stone, although fossilised footprints have been found at Hilbre Island and at Storeton Quarry. A slab of rock from the quarry containing fossilised footprints can be seen at Birkenhead Priory.   These layers of sandstone, folded and cracked by later movements in the Earth's crust can best be seen at the cutting on the Heswall-West Kirby Road.  If you go and look, take great care.  The pavement is narrow there and the traffic fast. 
 
 
Click on the image to see a larger version.  The different bands are interpreted as showing wide (reddish) deposits of wind blown dunes and very narrow (cream) bands of flooding deposits when the area was covered in water from nearby rivers.
 
The sandstones are red because they include large amounts of Iron Oxide (common rust!) among their consituents.  Mainly the stone is formed by grains of quartz, which is Silicon Dioxide.  Quartz is the most common mineral in the earth's crust. 
 
The Lower Triassic (between 251 and 245 million years ago) sandstones of the Sherwood Sandstone Group form low, but prominent ridges on the Wirral Peninsula. The Sherwood Sandstone Group consists largely of red, yellow, and brown sandstones that often show colour mottling. Pebbles are scattered through much of the rock; the smoothness and roundness of the pebbles shows that they were transported by a large and powerful river system, probably on the edge of an dry, desert mountain range. Where they are close to the surface as at Thurstaston, they are covered by Podzolic or rather poor, well drained soils.  In turn this is the most suitable type of soil for heathland vegetation.
 
In the last 2 million years, the Earth has been in what is known as the Pleistocene period.  During this time there have been repeated ice ages with regular glaciation.  There is clear evidence in the parish of this because, overlying the sandstone in most of the parish, there are the remains of the last ice age whose glaciers retreated when the Earth warmed up about 12 or 11,000 years ago.  These glacial deposits can best be seen on the shores at Thurstaston where the clay cliffs are well exposed. 
 
 
 
If you go to look at these cliffs, again take great care and do not climb on them.  They are very unstable and can break up without warning.
 
This clay is formed from the rocks over which the glacier has travelled, all the while grinding them into smaller and smaller particles.  At the same time it the glaciers picked up pebbles and larger stones and mixed them into the clay.  The clay in the parish almost certainly includes stones picked up as far North as Scotland.  Because it is ground so small, the overlying vegetation on boulder clay can be particularly lush, hence the grassland, which in the past has supported dairy farming so well here.