The Earth Story

Siccar point by drone. Original caption:

PENULTIMATE GEOLOGY IN OUTCROP

I can think of no better title to accompany this photo. Please imagine this as a 48 point banner headline.

A picture is worth a thousand words; an outcrop like this is worth, well, at least about 300 million years of geologic history. The ability to “read” the geologic history of an area by viewing its structures and stratigraphy is a skill essential to a geologist, and this spectacular outcrop is a virtual banner headline.

This outcrop is from Telherio Beach, Portugal. Geology addicts of all degrees of education can make out the awesome unconformity above the folded formation. Let’s add just a minimal of information: The lower deformed unit is upper Carboniferous in age (the Brejeira Formation) and the overlying undeformed unit is Triassic (the new red sandstones of the Grés de Silves Formation).

Before the deposition of the sandstone, the Brejeira Formation was 1. Deposited, 2. Deformed in some immense compressional event, 3. Exposed to weathering. The timing suggests that the folding is related to the merging of continental blocks into the Pangaean super-continent. The lack of deformation in the Triassic certainly suggests that at this locality anyway, not much has happened since.

Like all headlines, the details are given in the accompanying article. There’s so much more that can be learned by detailed study of these formations, their composition, their strain… but for a headline, this is one of the world’s penultimate geologic outcrops.

If this is the headline, I wanna read the whole story!

Annie R

Photo: Gabriela Bruno,Telheiro Beach, Portugal. And thanks to Francisco Sousa for inspiring this post.

Read also: http://rop.ineti.pt/rop/images/intro/it2_en.html http://revistas.ucm.es/index.php/JIGE/article/view/33912/0

Siccar Point

The US Geological Survey’s Landsat 8 satellite captured this image of Siccar Point in Scotland, perhaps the most famous angular unconformity on Earth. At this site, naturalist James Hutton recognized that the presence of an angular unconformity required millions of years of Earth history – the first layer of rocks had to be formed, tilted, and eroded, before another layer could be deposited on top. What I particularly like about this image is that if you look along the shoreline to the west of Siccar point, you can actually see the pattern created by the dipping beds of the Old Red Sandstone. There are linear outcrops all along the shoreline that outline the intersection of those beds with the surface – that direction is what geologists call the strike. It even looks like one of the creeks follows that direction for a short distance. To the right of Siccar point, you can no longer see this pattern, as the older and steeper beds strike in a different direction.

JBB

For more on Siccar point, see our previous posts: https://tmblr.co/Zyv2Js11Z9mNF https://tmblr.co/Zyv2Js1R6HJsL https://tmblr.co/Zyv2Js221GUwo

Image credit: https://earthobservatory.nasa.gov/images/92598/the-abyss-of-time

Siccar Point: where modern geology started.

Located on the East coast of Scotland, this location is the most important site in the history of geology. James Hutton, widely seen as our founding father (see http://tinyurl.com/kwz2543) had a revelatory moment here in 1788 that opened his eyes to the existence of deep time, recognising in a flash of inspiration the story that these rocks had to tell us. He used this inspiration to prove his uniformitarian theory, that processes had remained the same throughout geological history and that immense cycles of time were represented by the rock record that was then being explored and discovered for the first time.

At this promontory in Berwickshire, one the site of an ancient hill fort, an angular unconformity, recording a time in geological history when no rocks were deposited because erosion was occurring, reveals some pretty astounding facts of Scottish geology. Here, near horizontal layers of 345 million year old Devonian old red sandstone overlay highly folded, near vertical, 425 Ma Silurian greywackes. At the junction, a basal conglomerate containing clasts of greywacke testifies to the erosion that preceded the deposition of the land sedimented desert sandstone. Higher up the cliff the conglomerate is absent, indicating palaeotopography, where a Devonian hill and valley sat side by side, with the hill shedding the clasts into the valley below where they eventually formed the pudding stone.

The greywackes were deposited off an ancient coastline into the Iapetus Ocean, and were once also horizontal. As this ancient sea gradually closed during the formation of the supercontinent Pangaea they were folded, thrust up and tilted to their current near vertical state. Six separate events of folding and uplifting have been discerned in the rocks. The mountain range then gradually eroded before the great Pangaean desert formed, depositing the aeolian (wind borne) desert sandstone above the already ancient and tilted rocks.

Hutton presented his findings in his books Annals of the Former World and Theory of the Earth, in which he stated "From the present state of things, we have it in our power to reason from effect to cause, and read the annals of a former earth", and while discussing his new vision of deep time "I see no vestige of a beginning (of our world), no prospect of an end". The site has now been designated a site of special scientific interest, and remains a pilgrimage for university students and geologists worldwide.

Loz

Image credit: David Souza

http://www.geopoem.com/2013/06/unconformities.html http://www.electricscotland.com/mcintyre/index_f/menu_f/geology_f/siccar_point.htm http://all-geo.org/highlyallochthonous/2011/01/the-making-of-an-angular-unconformity-huttons-unconformity-at-siccar-point/ http://www.geos.ed.ac.uk/undergraduate/field/siccarpoint/ http://www.geowalks.co.uk/isiccar.html http://www.geowalks.co.uk/isiccartours.html

Ooh, the British Geological Survey presents this excellent video of a trip to Siccar Point, home of Hutton’s Unconformity and one of the most famous outcrops in geoscience. This outcrop helped forge the idea of geologic time.

Siccar Point, Scotland Scotland should hold a place of reverence in the heart of everyone who works in the Geosciences. Another of Scotland’s geologic gems; this outcrop literally led to the science of geology. This is the famous outcrop at Siccar Point, where James Hutton first put together the idea of geologic time as incredibly long, as having “that we find no vestige of a beginning, no prospect of an end”. This outcrop is a classic example of an angular unconformity. Hutton reasoned that the formation of this outcrop required immense amounts of time, geologic time. The rocks in the lower layer were deposited as sediments, lithified, tilted, and eroded over immense time. After that, an entire new package of rocks was deposited on top. Out in the world we can see snapshots of some of these processes; rocks being tilted by faulting, sediments being deposited, rocks being eroded after uplift, but they all take enormous amounts of time. This outcrop simply can’t be formed quickly by any rapid process. Today’s election results will be available in a blink of an eye compared to the time represented by this outcrop, but the legacy of both will endure. -JBB Image credit: Anne Burgess http://en.wikipedia.org/wiki/Geology_of_Scotland#mediaviewer/File:Siccar_Point.jpg Read more: http://www.scottishgeology.com/geo/regional-geology/southern-uplands/siccar-point/ Have you been missing out on our posts lately? If so, it is due to changes in Facebook's filtering system. To find out how you can enjoy reading our posts more often click here: http://tinyurl.com/ll9wd7l.