Many thanks to Dr Roger Higgs, who hosts Geology walks along Bude beaches for providing the text and images.
The distinctive rocks known as the 'Bude Formation', covering much of north Devon and northernmost Cornwall, are best exposed in the magnificent cliffs. Studied by generations of geologists, and used as a geology training 'school' by universities and oil companies, the cliffs show alternating beds of sandstone and dark shale, deformed into zigzag folds; the upfolds are called 'anticlines'; downfolds are 'synclines'.
The rocks pose many fascinating questions. For example, how old is the Bude Formation? In what sort of environment (e.g. river, lake, sea, desert) were the original sand and mud layers deposited? How did such a thickness (about 1 km) of sediments accumulate? How did they harden to form 'sedimentary rock'? How were they folded?
We can answer these questions, because rocks 'speak' to geologists who learn the 'language'. In other words, the rocks are like a book, recording the local geological history.
Geologists tell the ages of rocks from fossils and from measurements of the radioactivity in 'igneous rocks' like granite and volcanic ash beds. The Bude Formation has few fossils and no igneous rocks. The scarce fossils, plus well-dated rocks just below and above, tell us that the Bude Formation was deposited about 300 million years ago, in the 'Carboniferous period', long before dinosaurs or mammals existed.
How do thick sediments accumulate?
The earth's surface can be divided into areas that are rising and therefore being eroded (like Cornwall today, and the Alps), and areas that are sinking ('subsiding') called 'sedimentary basins', like most seas and many river plains and deltas. In these basins, sediment layers ('strata') can accumulate to great thicknesses (km), keeping pace with subsidence. The 'Cornubian Basin', containing the Bude Formation and other (older and younger) deposits, covered much of Cornwall and Devon.
Environment of deposition?
We can interpret the Bude Formation as underwater deposits of a sea or lake, as it contains no evidence for dry land, like coal beds (compressed forest vegetation), fossil soils or fossil plant roots. (Note that the 'fossil forest' occasionally visible under the shifting sands of Crooklets Beach is FAR younger, less than a million years old; it lies 'unconformably' on the Bude Formation.)
A lake interpretation fits the Bude Formation best, because marine fossils ('goniatites') are absent, except in three very thin shale layers representing times when a good connection to the ocean existed, turning 'Lake Bude' briefly into a sea. Other fossils include a sardine-size fish with sharp teeth, unknown anywhere else on Earth, named after Cornwall and Bude, Cornuboniscus budensis; currently housed at the Natural History Museum in London.
How do sediments turn into rock?
With deep burial in a sedimentary basin, muds harden into shale by compaction, and sand grains become literally cemented together by minerals precipitated from groundwater, deep underground, over millions of years, forming sandstone. The hardness of the Bude Formation tells us that many kilometres of younger sediments must have been deposited on top; these have been eroded away (see below).
How do rocks become folded?
The Earth's surface consists of a number of 'tectonic plates' about 100 km thick. Plates move apart in some places, forming oceans by 'sea-floor spreading'; elsewhere plates collide, forming mountain chains; or they can slide alongside each other ('strike-slip' motion). This constant movement is called 'plate tectonics', which includes the older theory of 'continental drift'. The movement causes earthquakes on geological 'faults'; the largest faults are the plate boundaries themselves (the 2010 Haiti earthquake was of strike-slip type, caused by the North America Plate sliding past the Caribbean Plate). Over millions of years, plates can move thousands of kilometres, at about the speed our fingernails grow!
The Bude Formation became folded (literally squeezed) only 5-10 million years after it was deposited, when Africa and Europe, riding on separate plates, collided, near the very end of 'Carboniferous time'. This collision folded and uplifted the sediments of the Cornubian Basin, forming a mountain chain that ran east-west across Cornwall and Devon. In this way, a subsiding basin turned into an uplifting mountain range. The uplift led to erosion, mainly by rain and rivers, of the younger deposits off the top of the Bude Formation, exposing it as we see it today. In other words, the Bude Formation was once in the core of a mountain range that has since been almost entirely eroded away!
If you think THAT'S amazing...
At the time Lake Bude existed, Britain lay near the equator; since then, it has drifted (by plate tectonics) to its present position about 5,000 km north! Like Cornwall and Devon, the rest of Britain at that time was also mostly a subsiding basin, but largely covered by tropical forests. Burial and compaction of peat deposits made of dead trees produced our immense coal deposits; that's why this period is called 'Carboniferous'. The coal made Britain wealthy by providing the power for our Industrial Revolution.
So, we can imagine a warm Lake Bude in the south and rainforests in the north, much like the geography of the middle of Africa today. Geologists call the reconstruction of ancient environments, for any ancient time period, 'palaeogeography'.
Cornwall Wildlife Trust have several geological events throughout the year. Click here to see their events.