600 Million Years of Cynon Valley
Geological History

1. Introductory background

Large scale growth in population and development of the Cynon Valley happened in order to extract and use the mineral resources beneath the surface. To put that development in context, it is appropriate to outline the changes that have happened over time to that small part of the Earth's crust on which the Cynon Valley is now located and to demonstrate how those resources came into being. Since most readers of this account will not have studied the Earth Sciences beyond an elementary stage the following sections are intended to provide an adequate background in topics relevant to an understanding of the evolution of the Earth's crust in the specific vicinity of the Cynon Valley. Because there are now many sources of information on these topics available on the www, a few, which are considered to be accessible to pupils in secondary schools, are noted in the text.

1.1 The nature of the Earth’s crust

Since the late 1960s it has been accepted universally that the crust of the Earth is composed of several large masses of rock, referred to as plates, which are constantly in motion though be it very slowly, over the Earth’s mantle. A straight forward and simple explanation of the nature of the structure of the Earth can be found on:

https://www.earthlearningidea.com/PDF/196_Journey_centre_E.pdf

Whilst the detailed mechanism of plate movement is much debated, recent evidence suggests that the main mechanism is due to the pull of a plate composed of denser cold material sinking below a plate of lighter material which can, in turn, drive convection patterns within the Earth's mantle. This theory is known as Plate Tectonics and there are many web sites which provide an overview of this process accompanied by appropriate maps and cross sections of the Earth's crust. A highly acclaimed website, produced by the UK's professional geologist body, The Geological Society of London,

https://www.geolsoc.org.uk/Plate-Tectonics

has been produced for Key Stage 3 students and above, and is written in a manner understandable to non-specialists also. Another valuable site is:

https://www.earthlearningidea.com/PDF/326_Plate_driving_mechanisms.pdf

Beneath the oceans the Earth’s crust is of volcanic origin and is young, with new volcanic rocks being formed at the present day along submarine ridges where the crust is splitting, such as is taking place currently at Iceland. This crustal splitting in some regions is complemented in other regions by crustal collision and, where two crustal plates collide, the denser one sinks below the lighter, as is occurring at present day Indonesia. These elongated narrow parts of the Earth's crust are known as subduction zones. As part of this slow moving cycle those sediments deposited in seas and at ocean margins become rocks and under the pressures associated with the collision become deformed and fractured and, at depth, greatly heated also. Eventually, because they are lighter than the oceanic crust, they rise to form mountain chains as along the western seaboard of the Americas. These mountain chains in turn become a part of the stable continental masses some of which have cores of ancient rocks. This process has been taking place on the Earth for at least 4 billion years (the first 600 million years of the Earth's history may have been a little different on a very young planet) with continents colliding and splitting apart in roughly 650 million year cycles to produce the continents and ocean basins of the Earth as we know it today.

1.2 Geological Time

As historians have produced a time line for a developing history of mankind based on documentary evidence, and archaeologists have taken this further back in time based on remains and artefacts, geologist have produced a time line for the 4.65 billion years of earth history based on the information contained within the rocks of the Earth's crust. Whilst calendars give historians accurate dates on which to base their studies, for convenience of reference, groupings of events have been arbitrarily defined often in terms of a particular monarch's reign such as Tudor period (UK) or Ming (China) with a great degree of chronological accuracy. Where detailed evidence is less clear, broader divisions with less clearly defined limits have been established such as pre-Inca in South America, or, with even less certainty for earlier times, the Middle Neolithic.

The case is similar for the division of geological time. Recent geological events and the formation of those rocks can be defined with great accuracy especially if they have occurred since humans have been recording such data in official documents. The volcanic rocks formed by the eruption of Surtsey on Iceland, recent lava flows on Sicily or the violent eruptions in Indonesia such as Krakatoa are very clearly time defined and recorded. However, many of the rocks formed on those young features have already been removed by erosion and deposited elsewhere, thus showing how the record of even recent geological events can soon become complex and incomplete. Historians can probably relate to this scenario with the destruction of records during wars and revolutions or deliberately being destroyed to hide the actions (or inactions) of individuals in power for self preservation.

Rocks are the geologists’ “documents” and the further back in time one investigates the less complete the record may be and the less clear the rocks are to read. This is because the surface of the Earth is constantly being eroded with the fragments thus produced transported and deposited eventually, often, but not always, at a distance from the parent rock, to form new and hence younger rocks. The story of those secondary rocks begins with the formation of the sediment but, because of the durability of some minerals, it is possible sometimes to deduce something about those older rocks from which the sediment was derived.

Divisions of time for the most ancient rocks are in the order of 400 million years but for more recent ones divisions in the order of 100 thousand years can be achieved. For the last Ice Age deposits, an even greater level of accuracy can be produced. Whilst time progresses at a uniform rate, the formation of rocks is far from uniform. Some large bodies of rock, such as lava flows or mass submarine slides down continental slopes, can be formed rapidly whereas others can be very slow such as the formation of lime muds by precipitation in clear continental shelf seas as around the Bahamas at the present day. Geologists are aware of this and realise that the thickness of a mass of rock need not be directly proportional to the time taken for its formation.

The establishment of the geological history of an area can be compared with a historian producing a history of a period where few documents exist and those that do exist will have been produced by different people, written in different languages, each with its different focus or bias and the writers will have been very selective as to what was recorded. To add to the historian's problem some of those texts will have had sections removed, probably have been damaged by fire or water and all will have suffered the effects of age. The quality of the history produced will depend not only on the degree and certainty of the documents and the level of understanding of what has been written by the historian, but also on the consensus of thinking at that time and the influence of current attitudes to the past. A similar process is applied to the record in the rocks and the interpretations of that evidence will be modified at times by an increase in knowledge and understanding. Conjecture plays a role in both scenarios.

It is in this context that the following report on the geological history of the Cynon Valley is made and, for ease of communication, readers should make reference to a copy of the geological time scale for the past 560 million years which is the time period that is of relevance to this area. A very useful example, produced by the British Geological Survey, is available on:

https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/geological-timechart/

The initial diagram, (Figure 1), is adequate for this account and it is the right hand column and the dates before present (bp) that are of relevance. For those who are interested in more detail there are several sub windows on the web site that give that information.

 

1.3 Ancient Geographies

In order to appreciate the differences between the world as it is now and the constructions of the world at those different times in the past referred to in this account, and known technically as palaeogeographies, there are a number of web sites with maps that show the surface of the Earth at successive periods from the late Pre-cambrian (650 million years) to the present and some even show predictive models of the distribution of continents and oceans up to 250 million years into the future. A particularly clear and interesting set of maps have been produced by Christopher R. Scotese and can be viewed on

http://www.scotese.com/earth.htm. Most have been reproduced, where relevant, in the body of this report. Scotese has even produced animated versions of the changing surface of the Earth though time an example of which can be accessed on YouTube:-

https://www.youtube.com/watch?v=ZdOig3UkufE

To trace the South Wales area on these maps look initially for Avalonia or England because until the end of the Silurian period most of Wales had not been created. Later maps clearly show in outline the southern half of the British Isles but not Scotland because until the mid Devonian that was evolving along the American side of the intervening ocean on the edge of an ancient continent called Laurentia by geologists.