Saturday, January 30, 2021

Unconformities - The Evidence of Absence

Another post that is neither one thing or another.  An absence of structure.  Fragments on the page.  My apologies.

The Book of Unconformities – Not A Review

This post was originally prompted by anthropologist Hugh Raffles’ new book The Book of Unconformities:  Speculations on Lost Time (2020), but that post remains mostly unwritten because I’m really not sure what to make of the book.  Here are some fragments that might have gone into that intended post.

The book is a challenging amalgam of personal, anthropological, and geological history.  All of these?  Not really any of them?

The snare for me was his central metaphor:  geological unconformities.  These fascinate me, but very little of the geology treated by Raffles has anything to do with unconformities.

He defines an unconformity as “a physical representation of a gap in the geological record, a material sign of a break in time, readily readable once you know here and how to look.”

Raffles takes this idea of a rift in time into the social and cultural histories he recounts.

The book consists of seven chapters, each grounded in a different kind of rock and each delineating disheartening and disquieting social and cultural dislocations (e.g., the Lenape Indians living on Manhattan brutally displaced by Europeans, or the Inughuit of Greenland who were exploited and dehumanized by Robert E. Peary).

The immediate context for his nearly quarter century exploration of these stories of rocks and people is a search for an “anchor” in an “unmoored world” that had a sharp rift caused by the unexpected and tragic deaths three months apart of two of his sisters in the mid 1990s.  He writes, “[A]lthough my sister’s deaths were only minor horrors in the history of the world, for those closest to them even minor horrors transform all that follows.”

In this light, is it significant that Raffles describes an unconformity as “a cleft that can’t be closed”?  Is this book an effort to heal that scar and, thus, doomed to failure from the start?

Raffles certainly marshals myriad words in seemingly endless sentences and paragraphs that engulf the reader in details.  It’s not stream of consciousness, it’s stream of research. 

Unconformities – Some Thoughts


The Facts on File Dictionary of Earth Science (2000) defines unconformity as:

A surface representing a period of nondeposition or erosion separating rocks of different ages.  Some unconformities show a marked angularity, the beds above and below the unconformity surface having different dips and strikes, where other unconformities can be detected only by paleontological means.  (p. 339.)

In essence, an unconformity is a gap in geologic time.  It is where layers of rock laid down in discontinuous time periods come into contact.  This definition identifies two possible causes of this gap:  the intervening layer was never deposited or it was deposited but subsequently eroded away.

Unconformities are a source of fascination for me because of the challenge they offer:  how to explain the absence of rock, those gaps in time that they exhibit.  In the history of geology, though, unconformities are possibly most significant for their claim on great periods of time, a death blow to young earth creationism.

Scotsman James Hutton (1726 - 1797), justifiably considered by some as the father of modern geology, saw in unconformities, confirmation of his hypothesis that cyclical geological processes on Earth occurred over vastly long periods.  He described geologic time as “of indefinite duration,” and “that we find no vestige of a beginning, no prospect of an end.”  (John McPhee, Basin and Range, 1980, p. 108.)

Probably the most famous unconformity is at Siccar Point, Scotland, on the North Sea.  In 1788, the phenomenon of unconformities came famously into stark relief (literally) for Hutton and colleagues mathematician John Playfair (1748-1819) and geologist James Hall (1761-1832).  They approached the point by boat and it’s an understatement to say they were blown away.  Playfair would later write,

On us who saw these phenomena for the first time, the impression made will not easily be forgotten. . . .  We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of a superincumbent ocean. . . .  The mind seemed to grow giddy by looking so far into the abyss of time.  (McPhee, p. 107-108.)

Pictured below is part of the unconformity at Siccar Point.  The vertically oriented sheets of rock are Silurian greywackes and shales, some 440 million years old.  The red rock resting on top of the vertical rock and sloping from right to left is Old Red Sandstone, 380 to 370 million years old.  The gap between them represents 70 million years of elapsed time.  The lowest portion of the red sandstone is a conglomerate of red sandstone with pieces of the gray Silurian rock that had been eroded during the time it was exposed.  (These dates are those cited by the British Geological Survey in its spectacular video titled Siccar Point:  Birthplace of Modern Geology.)

(This picture was taken by Dave Souza and is licensed under the Creative Commons Attribution-Share Alike 40 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.  It can be found on Wikimedia Commons.)

At Siccar Point, the orientation of the Silurian rock and unconformity between it and the Devonian sandstone is the product of multiple forces.  Hutton was among the first to make clear that these occurred in sequence over time, not in one fell swoop as though through some cataclysmic event.  Geologist Andrew Kerr describes these events as follows, noting that Playfair’s sense of teetering on the abyss of time was in recognition of what they meant:

The profound message of Siccar Point, where relationships show that one group of sedimentary rocks first must have been deposited, then twisted and uplifted and then eroded, before being submerged and buried by a second series of very different sedimentary rocks, and then together uplifted and tilted for a second time to be eroded anew, is most eloquently delivered by John Playfair.  (Classic Rock Tours 1.  Hutton’s Unconformity at Siccar Point, Scotland:  A Guide for Visiting the Shrine on the Abyss of Time.  Geoscience Canada, Volume 45, 2018, p. 39.)


Geologist John D. Glaser observes that considering unconformities on a broad geographic scale, such as a continent, fewer will be identified.  As the lens is applied to ever smaller areas, the gaps in the fossil record caused by the absence of sedimentary action or by erosion more readily emerge.  Traveling from Scotland to a place that I call home, Glaser notes that there are three broad ones in Maryland:

In Maryland, interruptions such as this occurred during the Permian and Jurassic periods, and during the Oligocene epoch.  These, then, are major unconformities in our local rock record.  (Collecting Fossils in Maryland, Educational Series No. 4, Maryland Geological Survey, revised 1995, p. 8.)

I suspect that a fossil collector in Maryland, particularly one who focuses on the state’s coastal plain which encompasses fossiliferous sites on the Potomac River and on the western side of the Chesapeake Bay (c'est moi), is most likely to become aware of the unconformity I consider the most striking and most obvious.  That is, the absence here of fossils from the Oligocene Epoch (33.9 to 23.03 million years ago).  Earth scientist Martin F. Schmidt, Jr. states categorically that “there are no Oligocene sediments in Maryland.”  (Maryland’s Geology, 1993, p. 109.)

 A geologic map of Maryland makes this abundantly clear, at least as far as exposed rock formations are concerned.  Consider this small portion of the key of the 1968 Geological Map of Maryland (Maryland Geological Survey):

This portion of the map provides the key for identifying exposed rock formations of Maryland's coastal plain by color and alpha code.  This is the only area in the state with Cenozoic Era formations.  The key jumps from the Eocene (56.0 to 33.9 million years ago) to the Miocene (23.03 to 5.33 million years ago).  More telling, perhaps, cross sections on Maryland geologic maps also show no Oligocene deposits.  [After the initial posting, this paragraph was edited to make clear that the geologic map key references rock formation exposures.]

Why no Oligocene sediments and, hence, no fossils from that epoch?  Geologist John Means, among others, explains this absence by sea level retreat which exposed this area, precluding deposits of Oligocene-aged sediment.  (Roadside Geology of Maryland, Delaware, and Washington, D.C., 2010, p 231.)  Further, he makes it clear that the coastal area of Maryland has experienced repeated sea level drops and rises.  It’s an ongoing process stretching back into deep time.  Hutton would have understood.

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