Tag Archives: Charles Lyell

All the Stratigraphies

The title page to Johann Lehmann’s 1756 publication on stratigraphy

Preamble: This post outlines the development of formal stratigraphy, focusing on Chronostratigraphy. Entire books have been devoted to this topic; one of the most entertaining and enlightening is Martin Rudwick’s excellent account of the conduct of 19^th century gentlemen who argued the case for the Devonian. My account is woefully brief, but there you have it…

This is part of the How To…series on Stratigraphy and Sequence Stratigraphy

The role of a stratigrapher is to decipher the organisation and origin of strata, and the order of events recorded therein. Stratigraphy is all about space and time.

The idea that strata might be ordered in a predictable manner dates back centuries. Imagine an 18^th century surveyor or land-man wandering the hills in search of coal, and observing seams that were associated with sandstone, mudstone and limestone, and that the arrangement of these strata was the same from one locality to another. Finding a particular limestone might lead the surveyor to predict the presence of coal seams, if not at the surface, then beneath it.

One of the earliest attempts at formal stratigraphic classification was Johann Lehmann’s three-fold subdivision of rocks, published in 1756. His scheme combined a description of strata, placing them in order, oldest at the bottom:

Primitive, or Primary crystalline rocks, lacking fossils.
Secondary stratified, fossiliferous rocks (some derived from the Primitives), and
Surficial, unconsolidated alluvium and colluvium.

The concept of relative time encapsulated by this succession was based on Steno’s axioms. Towards the end of the 18^th century, the influential German geologist Abraham Werner expanded Lehmann’s classification, claiming it to be global in extent, to include a Primitive (crystalline) Series, a Transitional Series (greywackes, limestones), a Stratified, or Secondary Series (much like Lehmann’s), an Alluvial Series to which the term Tertiary was applied by some, and a Volcanic Series. Like Lehmann, Werner’s scheme also combined the physical attributes of rocks and their relative age. Werner is also remembered for advocating that all rocks had precipitated at different times from a universal ocean – a concept that came to be known as Neptunism.

Throughout late 18^th and 19^th centuries, the naming of rock units became widespread in Europe, Britain, and North America, usually beginning with local knowledge of commonly occurring successions that were traced and mapped farther afield. Fossils became an important part of this development, where it was recognized that certain taxa and assemblages of fossils were characteristic of particular stratigraphic intervals. In England, this was exemplified by William Smith’s iconic map. Assemblages of rocks having identifiable lithological and fossil characteristics were given names that are still in use today; the progress of mapping demonstrated the repetition of characteristic successions, or Groups of strata. Thus, Old Red Sandstone was overlain by Mountain Limestone followed by Coal Measures; the group was subsequently called the Carboniferous Order by Conybeare and Phillips in 1822 (Phillips, 1837), and later the Carboniferous System.

The first half of the 19^th century saw a proliferation of Systems, each containing characteristic rocks and fossils, where the relative time relationships of fossils are strategic. In England, and no doubt elsewhere, there were competing views on the placement of system boundaries, and whether systems once established, could be split – some of these rivalries became bitter, even publicly so (Rudwick, 1985). For example, the Carboniferous was eventually divided into Devonian (based on the succession in Devon) followed by Carboniferous, the Cambrian was inserted between Primary rocks and the Silurian (by Roderick Murchison), and later the Ordovician (Charles Lapworth) between these two Systems.

It is important to remember that prior to Charles Darwin (1859) and Alfred Wallace, there was no accepted scientific theory that explained the transition from one species to another – the observation that species change through successive strata does not require a biological mechanism – just a good pair of eyes. Of course, post-Darwin and Wallace, the underlying theoretical mindset changed.

By the late 19^th century all major system names, as we know them, had been proposed:

Sedgwick – Cambrian, 1835
Lapworth – Ordovician, 1879
Murchison – Silurian, 1835
Murchison & Sedgwick – Devonian, 1839
Conybeare & Phillips – Carboniferous, 1822
Murchison – Permian, 1841
Von Alberti – Triassic, 1834
Von Humboldt – Jurassic, 1799
D’Halloy – Cretaceous, 1841
Aduino – Tertiary, 1760
Denoyers – Quaternary, 1829

Charles Lyell (1833) created a similar scheme for Tertiary strata, giving us Pliocene, Miocene and Eocene, based on the more modern appearance of fossils; characteristics that clearly set them apart from all Secondary strata. Based on the distinctiveness of fossils in each of Werner’s Transitional, Secondary, and Tertiary Series, Lyell coined the now familiar terms:

Cainozoic (Tertiary) fossils have modern affinities, mammals, and the appearance of Homo sapiens,

Mesozoic (Secondary) the proliferation of ammonites and dinosaurs,

Paleozoic (Transitional) ancient life forms: first invertebrates, first fish, first plants,

Azoic (Primitive)

Publications by John Phillip and Charles Lyell in 1841 extended the system of classification to the entire geological column. Phillips used Lyell’s earlier divisions and subdivided them into Lower, Middle and Upper. Lyell went one further, and referred to the Primary, Palaeozoic, and Tertiary as Periods, acknowledging their chronostratigraphic significance.

Development of a more formal system of Eras and Systems as fundamental units of geological (relative) time was based entirely on fossils. An early proponent of this strategy was James Dana who in 1880 referred to the Cenozoic (Cainozoic), Mesozoic, Paleozoic and Archean as Times, each Time consisting of Eras (Devonian etc.), divisions within each Era such as Early, Middle and Late were called Periods, and subdivision of Periods into Epochs. To complete the duality of the Geological Time Scale, Dana applied the corresponding terms Series, Systems, Groups and Stages respectively to the equivalent rock units (Dana, 1894). Dana’s time scale has a particularly modern look to it.

Dana’s Geological Time Scale, and those of other geologists, have in the intervening 140 years, gone through many iterations, modifications, additions and subtractions. One of the most important contributions was the addition of radiometrically determined ages, that changed the relative time scale based on fossils and superposition, to a numerical scale. Geologists could now talk of events 200 million years ago (200 Ma) in addition to referring to Early or Lower Jurassic.

In 1974 the International Commission on Stratigraphy was established, with the aim of standardizing system boundaries. The modern Time Scale moved Dana’s time units to the left – the Devonian is now a Period/System, and the corresponding Era (time) is the Paleozoic. There is now reasonable international consensus on the format and content of the Geological Time Scale. However, the entire enterprise is continually under review; witness the debate on the Anthropocene, the geologically youngest, human-influenced epoch.

We now employ three primary components in formal stratigraphy:

Time units (Era, Period) that refer only to geological time and not process.
Time-rock units; strata deposited during a specific interval of time (System, Epoch).
Rock Units that refer only to the composition and mappability of strata (Formations, Groups).

The International Chronostratigraphic time scale provides the framework for more local stratigraphic schemes. For example, much of the New Zealand Cenozoic time scale is based on marine faunas that have little correspondence with northern Hemisphere taxa. Therefore, the New Zealand scheme has Series and Stages, superimposed on International Ages and Epochs.

Formal stratigraphic schemes

Lithostratigraphy

Formal lithostratigraphy is concerned with the description and mappability of rocks, using physical, fossil, and mineralogical attributes. The basic lithostratigraphic unit is the Formation. A Formation must be mappable; it must have well defined and easily identifiable surface or subsurface contacts. Contacts can be abrupt or gradational; they may be unconformable, but an unconformity should not pass through a formation. There is no reference to time. Formations can be subdivided into Members or smaller entities like Beds. Two or more Formations can be named as a Group.

The boundaries of Formations are invariably diachronous. The definition does not include any interpretive quality. Formations require Type Sections, that are locations or outcrops that contain most of the unit’s defining characteristics. Type Sections should be accessible.

Formations need not consist of a single lithology; interbedding of two or more lithologies is common. What is important is the internal consistency of the interbedded nature of strata compared with underlying or overlying formations. The geometry of lithostratigraphic units is also highly variable: sheets, lenses, shoestring, splits, and pinch-outs.

Chronostratigraphy

Prior to the advent of radiometric dating (early 20^th century), the concept of geological time was based on superposition and the observed transitions from one taxon to another (fossils); this was relative time, where stratum A is older than overlying stratum B. While the immensity of time was in little doubt (at least among the 19^th century scientific community), there was no way to quantify it in terms of years. Intelligent estimates were made based on biblical genealogy (Ussher, Kepler), the rate of Earth surface processes (Lyell, Darwin), and the rate of Earth cooling (Comte de Buffon, Lord Kelvin).

One of the first attempts to radiometrically date rocks (using isotopes of uranium) was made by B. Boltwood in 1907, a Yale University professor. Since then the science and technology of radiometric dating has advanced to the point where minute zones in a single zircon crystal can be dated.

The evolution of chronostratigraphic time scales is noted above. Radiometric dating has provided dates for all the important time and time-rock subdivisions.

Biostratigraphy

The chronological ordering of strata is based on two principles: superposition and the observed stratigraphic variations in fossils and fossil assemblages. These principles provide the foundations for the stratigraphic time scales we use today. The principle of faunal succession is based primarily on the appearance of specific organisms in certain strata that, in progressively younger rocks (deemed younger because they occur higher in the stratal succession), morph into different, but related organisms.

An early attempt to systematically apply variations in species was published by Albert Oppel (1856-1858), who described successive changes in ammonites, beginning with the lowest and oldest Psiloceras Planorbis. He proposed 14 Zones within the Lower Jurassic Lias of England and Germany. Oppel observed that the range of various species was variable; some had narrow stratigraphic ranges, others ranged over greater thicknesses of strata. However, all could be regarded as temporary. He defined a zone by the first appearance of a species, the overlying zone by the appearance of a new species and so on. Any species may range through more than one zone, but its first appearance was considered unique. The concept of zone, or biozone forms the basis of modern biostratigraphy.

Biozones (zones) are the fundamental biostratigraphic units. The five formal biozones are: range zones, interval zones, assemblage zones, abundance zones, and lineage zones. Each biozone is distinct. The International Stratigraphic Commission, and the North American Stratigraphic Code provide complete definitions and rules for their usage.

Magnetostratigraphy

Earth’s magnetic field is generated by a hot (4000-5000^oC), fluid-like, iron-nickel rich outer core that moves slowly around a solid iron inner core. The shape of the magnetic field is approximated in a simple experiment using a bar magnet and iron filings. The magnetic field is vertical at the poles and horizontal at the equator. Between the magnet poles, the iron filings approximate lines of equal magnetic intensity. In a three-dimensional Earth, we know that magnetic North moves in a circuitous path around the Geographic North Pole (in fact it has moved more than 1000 km since 1831 when first measured by James Ross). We need two measurable parameters to define the orientation of the magnetic field at any point on Earth’s surface:

The angle between magnetic north and Geographic north – the declination, and
Inclination (or magnetic dip), the up or down angle of the field measured from a horizontal plane at any point on the surface. This is approximated by a compass needle pointing down in the northern hemisphere (positive dip), and upwards in the southern hemisphere (most commercial compasses don’t show this because they have been balanced).

Has magnetic polarity reversed in times past?

A couple of early 20th Century geophysicists, Bernard Brunhes and Motonori Matuyama, devised experiments where remnant magnetism was measured in volcanic rocks. The rationale is that when lavas solidify, iron-bearing minerals in the rock, especially basalt, will act like tiny fossil magnets that record the direction (polarity) of the magnetic field at that time. Their experiments demonstrated that the magnetic field had indeed reversed in the distant past. If the age of the rock being measured is known, then so too is the age of the reversed magnetism, a method now extended to sedimentary and other kinds of volcanic rock. It has since been established that reversal of Earth’s magnetic field has occurred every 200,000 to 300,000 years over the last few million years. The last reversal took place 780,000 years ago; this is called the Brunhes-Matuyama Reversal.

The formal stratigraphic measure is the magnetostratigraphic polarity unit. The unit is recorded as Normal Polarity (north pointing) or Reversed Polarity for a body of rock. According to the International Commission on Stratigraphy, a polarity unit is:

a single polarity of magnetization;
an intricate alternation of normal and reversed polarity of magnetization;
having dominantly either normal or reversed polarity, but with minor intervals of the opposite polarity.

An example is shown on the New Zealand stratigraphic chart above.

References

J.D. Dana, 1894. Manual of Geology: Treating of the Principles of the Science with Special Reference to American Geological History. 4^th Edition. American Book Company, 1088 p.

J. Lehmann, 1756. Versuch einer Geschichte von Flötz-Gebürgen (Attempt at a History of Stratified Mountains). Berlin. Available to read at Linda Hall Library

Lyell, 1833. Principles of Geology. John Murray, London.

Lyell, 1868. Elements of Geology, or the ancient changes of the Earth and its inhabitants as illustrated by geological monuments. Appleton and Company. 803p.

A. Oppel, 1856-1858. Die Juraformation, Englands Frankreichs und des Sudwestlichen Deutschlands. The manuscript is available, in German

J. Phillips, 1837, Treatise on Geology. Longman, Orme, Brown, Longman, London. The text can be read in digital form on Internet Archive

J. Phillips, 1841. Figures and descriptions of the Palaeozoic fossils of Cornwall, Devon and west Sommerset. Longman, Brown, Green, and Longmans, London. Digital version available

M.J.S. Rudwick, 1985. The Great Devonian Controversy: The shaping of scientific knowledge among gentlemanly specialists. The University of Chicago Press, 494 p.

H.S. Williams, 1893. The making of the Geological time scale. Journal of Geology, v. 1, p. 180-197.

A timeline of stratigraphic principles; 19th C-1950

William Smith (1769-1839)

William Smiths map published in 1815 – the one that Changed the World: https://www.nhm.ac.uk/discover/first-geological-map-of-britain.html

“It was the work of genius, and at the same time a lonely and potentially soul-destroying project. It was the work of one man, with one idea, bent upon the all-encompassing mission of making a geological map of England and Wales” (quoted from Simon Winchester’s excellent account, 2001, p.195). The Map, the one that changed the world, was published on August 1, 1815. Four hundred coloured copies were printed, of which only a few remain. It almost covers an entire wall at about 2.6m high, and 1.9m wide. Its compilation required years of exhausting and costly field work – Smith was not a wealthy man, nor of gentlemanly status. He covered more than 130,000 square kilometres mapping and measuring, collecting thousands of fossils. His rock units, many of which were well known, were defined by lithology and fossil content – fossils were particularly important for correlation. Much of the original work was undertaken while surveying new canals.

The beauty of the Map lies in its artistry, the intellect behind its construction, and the important scientific and technological advantages of being able to see, at a glance, the geology of an entire county or country. I have seen one of the surviving copies in the Geological Society (London) foyer. It is stunning.

Upon publication, William Smith was celebrated, at least for a brief period. But the undercurrent of insults and condescension, and an even worse indignity, plagiarism, continued. However, a degree of fortune was to return in 1831 when he was celebrated as the first recipient of the Geological Society’s Wollaston Medal. In his delivery speech, Adam Sedgewick (in important personality in 19^th century English geology) even referred to him as the Father of English Geology.

Charles Lyell (1797-1875)

Frontispiece and title page to Lyell's Principles of Geology

Frontispiece (Roman Temple of Seraphis in Pozzuoli) and title page to Lyell’s Principles of Geology, in the 11th Edition 1872.

Lyell was a quintessential Victorian gentleman (later a Baron), lawyer, and the most influential geologist of the time. He championed Hutton’s uniformitarianism and deep time, providing a Gradualist’s antidote to the prevailing Catastrophism. For Lyell (and Hutton) the upheavals and contortions of the Earth required time beyond reckoning. Although he could remove himself from these Catastrophist reckons, he was acutely aware that some Earth processes can occur rapidly in a geological sense. The frontispiece to Volume 1, the Roman Temple of Seraphis in Pozzuoli (near Naples) illustrates his ability to decipher geological events; the temple columns, partially submerged in 1828 when he observed them, contain clusters of bivalves about 2.7m above the water line. He deduced that the columns must have been almost fully submerged, and later uplifted. In fact, today they are more than 3m above the water line.

His first edition of The Principles of Geology was in 3 volumes, the 1^st edition published 1830-33. They constitute one of the first set of texts that deals with pretty well everything geological – from sedimentary to igneous and volcanic processes, earthquakes, the uplift of mountains and the erosion that wore them down, fossils, climate, past glaciations. These weighty tomes were hugely influential to young Charles Darwin who obtained copies during his time on the Beagle (December 1831 to October 1836. Lyell was later to become a good friend and arm-twister to Darwin.

Elizabeth Carne (1817-1873)

Carne combined banking and philanthropy with her love of geology, the latter undertaken in Cornwall. She published four papers in the Royal Geological Society of Cornwall Transactions (she was the first female member elected to this society), including one that reveals a description of ancient sea levels; Cliff Boulders and the Former Condition of the Land and Sea in the Land’s End district. She identified features of the boulders that were like modern deposits along the Land’s End shore, but elevated well above the present shoreline. She was well read, but I cannot ascertain whether she was conversant with the Hutton or Lyell texts. Regardless, her interpretation of the boulders as an ancient beach, formed at a different relative sea level, shows interpretive skills that extended beyond the standard diluvian dogma.

Florence Bascom (1862-1945)

Florence Bascom’s contributions to geology and stratigraphy were numerous, but key among them were her pioneering efforts in developing microscopy as a tool for unraveling rock histories (petrology), and a love of teaching that culminated with her founding the geology department at Bryn Mawr College in Pennsylvania. She applied the principles of crystallography and petrography to sedimentary rocks, having learned the skills studying crystalline rocks. An ability to extend these skills enabled her to decipher sediment provenance, and to distinguish between metamorphosed rocks and non-metamorphosed sediments (this was the topic of a dissertation). She became an expert in Appalachian geology. Microscopy in all its forms continues to play a vital role in many facets of geology.

She earned four degrees, the final one a PhD at Johns Hopkins University in 1893 – not only the first geology doctorate, but the first woman to earn a PhD there. Bascom was the first woman to be hired as a geologist by the U.S. Geological Survey, and the first woman to be elected to the Geological Society of America in 1924.

Johannes Walther (1860-1937)

Walther was a German geologist who gave us The Law of Correlation (or Succession) of Facies, published in 1894 (and translated from German by Gerald Middleton 1973). He became a student of science and geology at a young age (12-15), one of his early mentors being Ernst Haeckel, a biologist, a Darwinist, and brilliant illustrator of living and fossil organisms.

His investigations involved marine geology, comparisons of modern and ancient sedimentary processes (uniformitarianism), was one of the earliest to recognize bioerosion as a significant source of carbonate sediment, and established that coral reefs had rigid frameworks plus interstitial sediment. Later in life he became a professor and dean (Gischler, 2011).

We remember Walther primarily for his ‘Law’, that is an essential part of any modern analysis of sedimentary facies and depositional systems (most English-speaking students will only be familiar with Middleton’s translation and commentary.

‘‘. . . only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time’’ (Walther 1894). Stated another way Walther’s Law indicates that facies that form coevally in laterally contiguous environments can be superposed vertically. It is possible that Walther was pre-empted by Lavoisier’s analysis, but Walther was the first to formally state the condition for facies disposition.

Johannes Walther's correlation of facies

An illustration of Walther’s Law. The deposits encountered on a transect across the shelf are represented stratigraphically in the coarsening upward succession

The example shown depicts a traverse from beach to outer shelf, with concomitant changes in sediment (particularly grain size and sorting), benthic fauna and flora, and sedimentary structures. Assuming no significant interruption, the facies progression will be translated to a hypothetical stratigraphy, in this case under conditions of regression and seaward migration of the shoreline.

Eliot Blackwelder (1880—1969)

Blackwelder was essentially a field geologist, known for his scrupulous attention to detail and observation, as broad-based in his interests as any Victorian gentleman naturalist. There are several reasons why his inclusion in the timeline is warranted, but key among them is a paper on unconformities published in 1909. He was the first to identify continent-wide successions of Phanerozoic strata, each separated by profound unconformities. We now refer to these successions as sequences.

Blackwelders unconformity bound successions

Blackwelder’s continent-wide unconformity bound successions and illustration of regression and transgression provided the conceptual impetus for later geologists like Larry Sloss, David Frazier, and Peter Vail

His Figure 2 is one of the earliest graphical portrayals of the chronostratigraphic significance of unconformities. The stratigraphic succession at A is continuous, but from B through C the duration of missing time increases, that he attributes to erosion of the rock record. The rise and fall of sea level (transgression and regression), and concomitant landward or seaward migration of the shoreline, produces a mappable contact that is diachronous. The diagram on the right is remarkably similar to Grabau’s 1906 Figure 7.

His scheme of unconformity-bound successions was expanded by Larry Sloss in 1949.

An excellent publication by Andrew Miall (2016) revisits Blackwelder’s unconformities, evaluating them in a more recent context.

Joseph Barrell (1869-1919)

Barrell’s interests extended from the global (heat production, isostasy) to the origin of sedimentary facies and depositional rhythms, or cycles. His classic paper on cyclicity (1917; a 159 page tome) considered stratigraphic rhythms and rates of denudation and sedimentation. He portrays these cycles in the context of changing baselevels, one of the first explicit explanations of the value of a datum in stratigraphy. His Figure 5, that could be transposed to any modern publication on changing sea levels, shows the effects on sedimentation from harmonic oscillations in baselevel.

Joseph Barrell's diagram of cycles and base level

Barrell’s 1917 diagram is a remarkably modern take on the relationship among base level cycles, deposition and non-deposition (diastems). Annotations in blue are mine. Modified from Geological Society of America Bulletin v.28

The diagram models:

The long-term change in baselevel caused by subsidence (curve A),
Long-period oscillations, or 2^nd order cycles caused by tectonism, and
Short period (higher order) cycles caused by fluctuating climate.

Higher order cycles are superimposed on long period cycles; we certainly know this to be true, for example Milankovitch astronomical cycles that drive oscillations in climate. Barrell then correlates changing baselevels with the deposition of sediment, and periods of little or no sedimentation that he calls diastems (D on the stratigraphic column) caused by a reduction in sediment supply and/or erosion. The corresponding time intervals are shown in the top bar – solid black lines represent sedimentation, blank spaces represent diastems.

Barrell’s diagram further illustrates:

Stratigraphic columns may appear to represent continuous sedimentation, but in fact they contain long periods with no sedimentation.
Whether sedimentation or diastemic breaks occur depends on the overall effect on baselevel of all three cycles, and
Sedimentation only occurs when the overall baselevel is rising. In other words, when there is space to put sediment. We now refer to this as accommodation space. In his own words “…the deposition of nearly all sediments occurs just below the local baselevel represented by wave base or river flood level, and is dependent on upward oscillations of baselevel or downward oscillations of the bottom, either of which makes room for sediments below baselevel.” (p. 747). Some of the jargon has changed, but this is essentially a modern statement about baselevel controls on sedimentation.

Amadeus William Grabau (1870-1946)

Grabau was an American paleontologist who, in addition to important work in continental USA, also spent a significant part of his life in China. Of his many books, three that deserve mention here are On the Classification of Sedimentary Rocks (1904), Principles of Stratigraphy (1913), and importantly Rhythm of the Ages (1940) where he elaborates on his pulsation theory (a theory introduced in 1933 at the International Geological Congress, Washington).

The Pulsation Theory attempts to explain the repetition through time of strata having similar sedimentary and paleontological attributes. Repetition of these successions of strata, more commonly known as cycles and cyclicity, resulted from successive marine transgressions and regressions (about 20 years after Barrell’s model if cyclicity). Grabau was one of the first exponents of Global Eustasy, where advancing seas were caused by expansion of the oceans, and vice versa retreating seas. From his correlations of Phanerozoic stratigraphy, he reconstructed continental paleogeographies that were also consistent with Alfred Wegener’s ideas on continental drift.

Grabau extended Walther’s Law, recognizing that there are gaps in time caused by sedimentary processes, particularly erosion during shoreline seaward retreat. He coined the term hiatus to describe the absence of a rock record between pulsations, or cycles. The diagram below from Figure 7 of a 1906 publication shows a stratigraphic (left) and corresponding chronostratigraphic representation; the similarity to more recent diagrammatic expressions of onlap and offlap is striking. Note that this also approximates a sea-level curve.

Grabau’s 1906 chronostratigraphic illustration of transgression and regression, separated by an hiatus. From Geological Society of America Bulletin, v.17.

References

J. Barrell, 1917. Rhythms and the measurements of geologic time. Geological Society of America Bulletin v.28, p. 745-904.

E. Blackwelder 1909. The valuation of uncomformities. Journal of Geology, v.17, p.289-299. Available for download

E. Gischler, 2011. Johannes Walther (1860–1937): More than the law of facies correlation. GSA Today, August, 2011, p. 12-13.

A.W. Grabau, 1906. Types of sedimentary overlap. Geological Society of America Bulletin, v.17, p. 567-636.

A.W. Grabau, 1940. Rhythm of the Ages. Peking: Henri Vetch, 561 p.

C. Lyell, 1872. Principles of Geology, Volume 1. Eleventh Edition. John Murray, London.

A.D. Miall, 2016. The valuation of unconformities. Earth Science Reviews, v. 163, p. 22-71

G.V. Middleton, 1973. Johannes Walther’s Law of the Correlation of Facies. Geological Society of America Bulletin, 84: 979–988.

S. Winchester, 2001. The Map That Changed the World: The tale of William Smith and the birth of a science. Viking, 338 pp.

How do we know which way is up #3. A philosophical interlude

How Geologists Interpret Ancient Environments. 3 A Philosophical Interlude

You are confronted by a rocky cliff and your geologist friend tells you that these rocks formed originally in rivers that flowed through a wooded valley. How did your buddy come to this conclusion?

Continue reading →

Ancient earth. 2 How old is it?

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How old is the Earth?

“In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.” (Douglas Adams The Restaurant at the end of the Universe)

The question “How old is the Earth?” has provided us with some pretty interesting, often rancorous and even divisive debates over the last few hundred years. For some the debate remains unresolved. The science of dating things has progressed hugely over the last century and we can now provide, with significant confidence both relative age (A is older than B) and quantitative ages (radiometric dating) for all manner of physical objects. Age dating of earth materials is not just an interesting academic exercise; it has provided us with the tools to help evaluate energy and mineral resources, to assess the risks from natural hazards, and to study past geological events as they may relate to our future well-being.

So where do we begin? Perhaps at the beginning. Present estimates for the Big Bang and the formation of the Universe are about 13.8 billion years ago. We now know that our own Solar System began to form about 4.6 billion years ago which means there is an hiatus of 9 billion years. What happened during that great ‘interregnum’ is another story (how many other Solar Systems?). Our tale begins 4.6 billion years ago because that is closest to home. In this post we will be focusing on the bottom end of the time-line shown below. Continue reading →

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