Seeing rocks slant; Unconformities; old and new


One of the most telling of all geological structures are Unconformities where new rocks lie on older ones , often at an angle as this diagram shows;


The most famous is Siccar Point which James Hutton discovered in  the 18th century and described here by Paul Braterman;

I have never been to Siccar Point but have seen many other unconformities and here are some I have seen recently. I conclude with layers of sand deposited in the December floods.

Steamboat Unconformity, Black Hills SD.


This unconformity lies in the northern Black Hills about 25 miles from Rapid City and suffers from a surfeit of students. The steeply dipping beds are 1,500 my Precambrian phyllites  and above are the almost horizontal Cambrian sandstones of the Deadwood formation  in the order of 500 my. A friend Mike often goes there and has a hand on each series and says “There is a billion years between my hands”, no doubt upsetting his creationist students, as I did.

Grand Canyon


This is almost the CLASSIC unconformity in the USA of almost horizontal Cambrian lying on older Precambrian of the Vishnu series – over 1,500 my. Above that is the long, almost horizontal sequence up to the Kaibab, which is Permian (280 my). It is an incredible walk down to the bottom of the Canyon to the Colorado River and then ascending again.The shortest route is the Bright Angel Trail from the south side. as you see in the picture below it is a long way down and the dry river bed in the centre is only half-way down. The Colorado River is hidden from view.  Guidebooks advise against walking it in a day but some do just that in August in temperatures of 115 degrees plus in less than nine hours.

494 You can follow the various formations for miles and below is a cross-section of the Canyon.



Yellowstone, Mammoth Hot Springs

Yellowstone is the oldest National Park in the world and is a fascinating place, even though over-commercialised. It is one vast volcano with a ginormous caldera. A future eruption is on the card. Sadly most crawl round in their cars and miss the best of walking away from the crowds. grizzlys, wolves and bisons are common and it is advisable to attach a bear bell to your rucsac and take pepper spray. It is said that you can identify grizzly scat by the presence of bearbells.

There are vast number of hotsprings but fortunately more people go to Old faithful than all the other springs put together. At the north of the park are Mammoth Springs which are highly active and always changing. The most interesting thing at Mammoth Hot Spring are the hot springs with several tiers of springs.



However looking east above the town you can see a fine unconformity of red Mesozoic beds lying unconformably on slightly older grey calcareous ones.This is not a patch on the previous two, but still dramatic – at least by British standards.



And so to England;

Ingleton Falls, Yorkshire

The finest mountain in the Yorkshire Dales is Ingleborough, though it is no great height. This shot taken in january 2015 shows the almost horizontal strata giving a fine succession of lower Carboniferous rocks, starting with limestone and then going to grits and shales (equivalent in age to Bowland shales). The photo is looking south-east from Chapel le Dale but five miles south are the Ingleton Falls ,


where Carboniferous Limestone (~350 my) lies unconformably on tilted Ordovician slates and greywackes of the Ingleton Group  (~420my). All the Devonian is missing!


Anthropocene/Carboniferous Unconformity

Recently, geologists and others reckon we should recognise a new geological sequence in which the main agent is human, and thus call it the Anthropocene. It only includes the last few thousand years lof geological time. Driving to Durham this week as we passed the ruins of Barnard Castle, dating from the latter part of the 12th century, and I thought “That’s an Anthropocene-Carboniferous unconformity”. With 320 million years of time between the strata!

And so we have a horizontal bedded anthropocene deposit of reworked Carboniferous sandstones lying on top of slightly tilted Namurian Millstone Grit (320my ). The castle is made up of reworked Carboniferous sandstone then deposited in a more orderly fashion!!

I have to admit to being slightly flippant, but this does show geological effects of humans and also the disparity of geological time.


Sand Deposits, December 2015

Yes, there is still deposition today and during the floods in Lancashire of December 2015, much sand was laid down. I often stop by a bridge over the River Wyre between Scorton and Dolphinholme while cycling. I see the changing seasons and varying volumes of water in the river. Today it was tranquil,but last month a raging torrent would have been where I stood and probably above my head. As the water receded it left a few inches of sand and silt, as is apparent in this photo. If you dug a trench down you would find a succession of layers of varying thickness  and most would lie horizontal on the lower ones.



In spring there are bluebells and snowdrops here. I was amazed to see one snowdrop just bursting into flower. It seemed to be making a brave statement against the rather torn-up vegetation around it. It is incredible to think that just weeks before water was surging around here and yet this tiny snowdrop survived.



And for all this, nature is never spent;

there lives the dearest freshness deep down things;

Gerard Manley Hopkins, God’s Grandeur



1 thought on “Seeing rocks slant; Unconformities; old and new

  1. Viv

    I enjoyed the enormous pleasure and privilege of a visit to the Grand Canyon last summer, and found the geology completely captivating. Worth knowing that for those of us not up to a trek down to the river and back (and lacking a mule!) there is a brilliant 4.6km trail along the edge of the South Rim, with marks every metre representing 1,000,000 years, display boards showing the formation of the geology of the area at critical periods in geological history, and samples of the rock types in every major layer from the Elves Chasm Gneiss all the way up to the Kaibab . The trail ends appropriately enough at the geology museum – and was almost deserted in the early morning, even in August



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