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The Geological History of Ceredigion; Formation of the layers of rocks under the sea; Earth movements from 395 Million years ago; Shaped Stones; Formation of Mineral veins; Animal Impressions; Trace Fossils; Fossils; The Ice Age; glaciers; Pebbles from the beach, Aberystwyth.

The geological history of Ceredigion consists of:

1 Formation of hundreds of layers of mudstone under the sea.

2 Movement of these layers to form mountains and hills.

3 Scouring of the surface and deposition of clays and rocks by glaciers and melt-water during the Ice-age.

Formation of the layers of rocks under the sea

During the Silurian Period (430 million years ago) most of Wales was under the sea and on the edge of a large continent.The land was almost barren. It consisted of rocks, gravel and silt that was easily transported into the sea by rivers and floods. These deposits settled on the floor of the shallow sea but occasionally vast quantities were washed over the submarine cliff into the deeper basin, fanning out to form hundreds of layers of stone, sand and mud, known as Turbidites.The finest particles - silt and mud - travelled the furthest and formed this part of Wales. They turned to stone and are now known as the Aberystwyth Grits.


The depth of the layers of mudstone is unknown. Two boreholes have been cut in this area to study geological deposits and heat flow from the centre of the earth.The hole at Glan-fred near Dolybont reached a depth of 404.22 metres (1326 feet). The Core of this bore hole which was removed as drilling progressed shows many layers of mud and sand. Some of the patterns in the layers are not fully understood by geologists.


A concretion is a compact mass of mineral matter, usually spherical or disc shaped and embedded in a host rock of different composition. They are formed by precipitation of mineral matter, mainly a Carbonate mineral such as Calcite, but sometimes form from iron oxide or hydroxide, or silica, about a nucleus such as a leaf or a piece of shell or bone. They can be from a few centimetres to 3 metres or more in diameter.

Flutes and Grooves

Currents of water created impressions on the bottom of the sea. Casts of these patterns formed in the next deposit of mud are often thought to be fossils. One specimen on display was thought to be a dinosaur's footprint, but is simply a group of overlapping flutes.Grooves made by objects dragged along the bottom of the sea are also preserved in this way.

Earth movements from 395 million years ago

The layers of mudstone took tens of thousands of years to form and during the next few million years movements of the earth's crust compressed, lifted and folded the horizontal layers leaving some vertical or even turned right over. These movements created mountains.

Formation of Mineral veins

Far below ground level, super heated water dissolved minerals such as lead. This water shot up to ground level in cracks in the earth's crust, breaking off pieces of rock as it went. As these solutions cooled, veins of mineral formed, along with quartz and angular rock. It is these veins that were excavated by the local lead miners.

Animal Impressions - Trace Fossils

When the mud was being deposited under the sea, there were a few animals who lived on or just below the surface of the mud. These left impressions of their activities, including evidence of browsing, for food and burrowing.

Surface trail done by a gastropod grazing. This impression is called nerities.

Honeycomb pattern of burrowing. This pattern is called Squamo Dictyon

Casts of impressions of shells which were bounced along the sea bottom are also found.

Liesegang Rings

This is a banded feature, which is thought to have formed by chemical reaction, diffusion and precipitation.


Fossils are extremely rare in the rocks around Aberystwyth. The only fossils to be found are Graptolites, Orthocones, and some casts of Brachipods. The Orthocones have only been found in couple of locations in Ceredigion. Graptolites are thought to be colonial animals.


For many millions of years it seems that very little happened to the geology of Ceredigion. About 2 millions years ago the temperature dropped and much ice was formed. The ice moved in glaciers, pushing stone before it, then deposition the stone as moraines. Several of these form the 'roads' which stretch out into Cardigan Bay (supposedly to Gantref Gwaelod - the lost lands of Welsh legends).As the ice melted, mud, clay and pebbles were transported from as far away as Northern Ireland and Scotland and dumped in the lowlands and below the sea.


Most of the beach consists of local mudstone and quartz, but a variety of other stones are present. Most of these were brought from the north (North Wales, Northern Ireland and Scotland) by glaciers.