Geology - Senni Beds

Note added 29/04/2009: Linked Figures and Maps are not yet available for these pages.

The hamlet of Heol Senni is built on slightly calcareous sandy and silty rocks laid down by a large river system flowing over a tropical floodplain, near the middle of the Devonian period about 390 million years ago. To put this in context, the Earth is about 4600 million years old – about the age of the rest of the Solar System. Life took about 1000 million years to get going and by the Devonian period was becoming established on land (fig 1.1. Britain in the Devonian). The dinosaurs didn’t arrive until the Triassic period, 230 million years ago, humans first came to Britain a mere 750,000 years ago and the last ice age ended about 10,000 years ago.

The rocks of the Senni valley have been officially named the Senni Beds, by Britain’s geologists, and the name of Senni is thus widely known because the rocks outcrop over large areas to the east.

The Senni Beds are interesting because they contain some early vascular plants - plants with stems and leaves - the fossils of which (fig 1.2. fossils from the Senni quarry) you can see in rubble on the floor of the Senni quarry ●1 (this is a protected site so please don’t break rocks or remove samples). More than fifty different plants have been identified by spores preserved in these rocks.

Much of the Senni Beds are stained a slightly purple-red by the iron mineral hematite which is formed only in warmer climates. Our soils are a rich red largely because they have inherited this mineral from their parent rock.

At least three distinct kinds of rocks can be found – a coarser sandy and silty rock representing deposition within the meandering channels of the ancient rivers, a finer rock representing deposition over the floodplain to each side of the rivers (an over-bank deposit) and a conglomerate which consists of pebbles of siltstone, evaporite and other easily weathered rocks cemented in a silica matrix.

Evaporites are rocks which are left behind when lakes and lagoons dry out, as must have happened frequently in this part of South Wales at the time. They consist largely of the minerals which were dissolved in the water but which are precipitated as solid rock when the water dries up – like lime-scale in a kettle. Thus evaporite rocks tend to include minerals like calcite (calcium carbonate), gypsum (calcium sulphate) and ordinary rock salt (sodium chloride) which are all quite easily dissolved by the rain.

All three of the Senni Bed rock types can be found exposed in the Senni quarry and in the walls of buildings around the valley (fig 1.3. the three rocks in the quarry).

The evaporite conglomerate (fig 1.4. ) is often seen as a “bubbly” rock in walls, full of rounded holes but breaking such rocks open reveals a solid conglomerate core. Here, the soluble evaporite and the weak siltstone has been weathered away at the rock surface to leave only the cementing matrix. The evaporite itself may have formed in cut-offs and coastal lagoons which have been re-eroded by the river and broken into pebbles, which have then been covered in river silt and consolidated into rock once more.

One of the reasons why plants tend to flourish on the walls around our valley (fig 1.5. a plant growing out of a wall) may be the nutrients released and the simple soils formed when these rocks weather, exposed on the sides and tops of the walls. The cavities formed by the weathered evaporite stones make excellent, sheltered sites for plants to become established and the silt and evaporate provide nutrients.

The finer, over-bank floodplain rock contains some calcareous layerscalcretes – which were once soils and in which calcium carbonate (the mineral which forms limestone) became concentrated by the surface evaporation of calcareous (lime-rich) soil water. This same process can be seen in many warmer countries today, including Turkey and Spain  (fig 1.6. calcrete in the Senni quarry).

There are some outcrops around the Senni valley which show evidence of the cross-bedding in the rocks laid down in the river channel. This bedding is the result of progressive deposition from the waters in the river channel as the channel itself migrated across its floodplain. (fig 1.7. cross bedding in the Senni Beds). The rocks also show fine ripple-marks formed on the bed of the river, and drying cracks where the ancient flood-plain surface dried out and was then buried by the next flood.