The New Forest 
Let's Go Geologizing (and fly a kite)!

Understanding what one is looking at on the ground adds greatly to the experience of touring the New Forest near Southampton. Because New Forest landforms are subtle, a keen eye is necessary to discern and decipher the clues of gravel shape, stream form, hill slope, and vegetation type. Winter is a great time to view the land as well as the landscape.

Geological Development of the New Forest
The area of the New Forest is underlain by chalk, laid down in Upper Cretaceous seas between 99 and 65 million years ago. This chalk bed dips to form a basin, which filled during the Eocene (ca. 56-34 million years ago) with sediments from transgressing seas, freshwater lakes and rivers. These strata were eventually uplifted and tilted towards the south, so that today there is a progression [like a tilted loaf of sliced bread with the top crust representing the overlying gravel and the stacked slice surfaces exposed] from north to south in the New Forest:

• the Lower Bagshot beds of deltaic origin
• the Bracklesham Beds of alluvial sand over clay
• the Barton Clay of marine origin
• the Barton Sand bed of loamy sand
• the Headon Beds of loamy clay and marine shell marls

In the Pleistocene (ca. 2 million to 10,000 years ago), these slanted beds were overlain by alluvial gravels, which now only survive on the high points of all the above geological beds.

New Forest Landscapes: Cliffs, Hilly Terraces, U-shaped Valleys
Piper’s Wait is billed as the highest point in the Forest. Surprisingly, it is not a mountain but one of nearly 150 named forest carparks, located on a high gravel terrace at the northern end of the Forest. Bordered on the north by the Bracklesham escarpment, the terrace represents the original, undissected surface of the Early Pleistocene floodplain gravels. The braided streams that laid down these gravels were early versions of the Avon and Solent Rivers.

Today the Avon flows down the western side of the Forest, having cut a spectacular river cliff affording views westward to chalk hills from the Castle Hill carpark. On the south, the Solent is now a submerged river valley forming the 1.2 –8 km wide sea passage between the New Forest and the Isle of Wight. In the Early Pleistocene it was an eastward flowing river that was capable of cutting terraces from west to east, flowing across a more extensive amount of land exposed by lowered sea levels and eventually into the river that drained the dry basin of the North Sea.

Erosional valley

From Piper’s Wait, about twelve such terraces drop down to the sea, cut and filled successively during the Pleistocene. Unlike the highest terrace gravels, the middle terraces bear angular flints, indicating they were deposited by steep gradient streams which flowed across chalk — a substance now exposed to the surface only to the north and west beyond the New Forest. These middle terraces were heavily dissected in the Mid- to Late Pleistocene by small streams, but their U-shaped valleys have since been filled up with erosional deposits housing small bogs.


From Grass ‘Lawns’ to Heathland to Woods
The gravels forming the surfaces of the higher points in the Forest can support little but broad Calluna heathland. Exposed Bagshot and Bracklesham layers are also acidic and nutritionally poor, again hosting little but heather, gorse and self-sown Scots Pine. But the marine Barton Clay and Headon Beds and the loamy Barton Sands

Ornamental woodland

are less acid, supporting the Ancient and Ornamental Woodland in the north-central part of the New Forest and allowing brown forest soils to develop. Wide-open grass lawns are also a feature on the valley bottom silty river gravels accumulated over the Barton Sands.

While bogs are often thought to occupy the lowest point in the landscape, a tussock bog has formed on the hillslopes above Picket Brook. The water feeding this hillside bog comes from seeps along a hilltop juncture of overlying porous sands on impermeable clays. The tussock bog vegetation on the hillside is entirely different from the bracken on the sands of the hilltop above.




Geomorphology in Action

Point Bar in Linford Brook

Linford Brook, flowing beside the Linford Bottom carpark, affords several lessons in landscape change. A stream bank has been cut on the concave curve, with a deep pool at its base, while in the convex curve, a point bar of sand deposits is developing. On both sides of the meandering stream are abandoned oxbows, not lakes but boggy areas with different vegetation than that of the valley floor. A short walk away on Picket Brook is a nick point, a sudden drop in the floor of the stream of half a metre that indicates the uppermost reach of current erosional forces.

Nick point on Picket Brook

Most visitors to the New Forest enjoy the trees and wide-open views from the heathland, but few look under their feet to the history of the land itself. It is a story beginning with the chalky sediments accumulating from Cretaceous Seas. The chalk was then covered by a succession of seabed and coastal sediments, which were uplifted as dry land. In the Pleistocene when the sea coast was south of the Isle of Wight, the Solent flowed across the Hampshire Basin towards the east, cutting the terraces of the New Forest area, stepping down from north to south.

References:
Chaffey, John (2009) “Geomorphology of the New Forest, Hampshire”. Fieldwalk handout.
Anon. (1986) The New Forest Landscape. CCP 220. Cheltenham, Glostershire: Countryside Commission.

Plutonic lunch: new understandings of Shap granite

Recently, we ate lunch at the Honest Lawyer pub outside Durham – an oxymoron for sure. Staring at me from the table was the belly of a possible volcano: a granite tabletop! It looked suspiciously like Shap granite from Cumbria, but unfortunately the quarry provenience was not known to the waitress.

As any volcanologist (but who else?) knows, granite is mainly formed in volcanic arcs where magma rises through the Earth's crust in 'diapirs';  most of these diapirs solidify as plutons 5 to 20km deep in the crust, but some make it to the surface where magma is extruded through volcanoes as lava or volcanic ash. The magma that doesn't make it out cools slowly in the chamber or pluton, allowing very large mineral crystals to grow. In this kind of pink granite, the large pink rectangular crystals are potassium (K-)feldspar, also known as orthoclase. Regular feldspar (plagioclase) forms the white crystals surrounding the large pink ones, while the grey bubble-like crystals are quartz, and the black grains may be biotite or amphibole.

Shap granite is featured as the Rock of the Month for December 2011 by the Open University. The Shap granite outcrop in the Lake District is intrusive into the Borrowdale Volcanic Group, which was once an island arc off the micro-continent of Avalonia in the Iapetus Ocean between 460 and 444 million years ago [1]. However, Shap dates later via processes quite different from the subduction zone granite emplacement described above. After Avalonia was accreted to Laurentia (North America), the area of the Iapetus suture in northern England (demarcated by the Solway tectonic line) was subject to magmatism on both north and south sides of the suture. It is thought that a period extensional tectonics, creating a pull-apart basin 21km wide, was able to generate higher temperatures that caused both mantle and crustal melts to rise into the upper crust [2]. These were emplaced in the subsequent period of Acadian Deformation between 400 and 390 million years ago. Shap granite, therefore, stands as the representative of a newly understood source of magma generation that is totally divorced from subduction tectonics.

Many buildings around England feature Shap granite floors or columns. So you can also walk on it  or bump into it as well as eat off it. Just keep those sharp eyes open for Shap and you will be rewarded by a trip to the center of the Earth (or at least 5 kilometers down where granite forms).

Oh yes, and the Honest Lawyer has great Eggs Benedict. Did I say lunch? Maybe it was brunch...

[1] Huff, WD; Bergström, & Kolata, DR (2010) "Ordovician explosive volcanism," pp. 13-28 in The Ordovician Earth System, ed. by FC Finney & WBM Berry. Geological Society of America Special Paper 466.

[2] Brown, PE; Ryan, PD; Soper, NJ & Woodcock, NH (2008) "The newer granite problem revisited: a trans-tensional origin for the early Devonian trans-suture suite". Geological Magazine 145.2:235-256.

gleeb visits Toronto: standing on Scotland

Scotland used to be part of North America – a continent called Laurentia. The opening of the northeastern Atlantic Ocean around 55 million years ago sundered Scotland from its homeland and left it attached to Albion, the isle of Britain. In Toronto, I was hoping to visit the homeland of Scotland by looking at the Canadian Shield, the 'original' landmass that was the core of Laurentia.
What I found was different!

When I first learned about the far northwestern Scottish rocks called the Lewisian metamorphic rocks, I was fascinated that they were 2 billion years old. I wanted to go walk on them. Later I visited the Royal Gorge in Colorado, which cuts deep into the Earth's crust, and I discovered that the rocks at the bottom of Royal Gorge were also about 2 billion years old. So, maybe also would be the Canadian Shield.

Walking on Canadian Shield rocks in
Village of Yorkville Park, Toronto

The name Canadian Shield comes from the flat roundish shape of the metamorphic rocks that form a craton, a continental nucleus that has existed since the Precambrian (before 542 million years ago). So in the Village of Yorkville Park in Toronto, they have established an artwork that recreates the feeling of the Canadian Shield. Segments of metamorphic rocks from the craton have been concreted together in a shield shape. These particular rocks may not be 2 billion years old, but you can get the feeling of walking on an ancient landscape here in the middle of the city. Find them just north of the busy Bloor upmarket shopping street on Cumberland, across from Old York Lane.

Sadly, the closest relatives of Lewisian Scotland lie not on the Canadian continent but on Greenland. So I did not fulfill my wish of standing on Scotland in Toronto. But thinking about the landscape in broad swathes certainly does give one a different perspective on a visit to a new country.

Geology Lectures at the Geological Society of London

For the past two years, Shell oil company has sponsored monthly talks at the Geological Society of London. These are open to the public and deal with many topics of current interest. Past titles have included:
"Icelandic volcanoes, interactions between volcanoes, ice and atmosphere"
"Challenged by carbon: geologists, the oil industry and climate change"
"A mole in London: tunnelling beneath the city for major transport and infrastructure projects"
"The Anthropocene: living in a new age"
"Returning carbon to nature: the geology of carbon capture and storage"

Wednesday's talk was on "Paleogene climate conundrums", looking at the a previous instance of rapid climate warming at the Paleocene/Eocene (PE) boundary. Dubbed the PETM (thermal maximum), the climate at that time (between 50-52 million years ago) resulted in a temperature rise of 5°C at high latitudes due to injection of massive amounts of carbon dioxide into the atmosphere. Included in the lecture were fossils of a 15-metre long snake that weighed 3 tons, which would have needed a minimum of 30°C climate to grow that large. The PETM is being studied for comparison with our current global warming, with the conclusion that the rate of carbon dioxide addition to the atmosphere is much faster today, disallowing time for the ocean to act as a buffer by absorbing extra carbon. Thus, it was implied that whatever happened then will happen faster and perhaps more extremely now.

Talks for the rest of this calendar year are:
19 October "Earth's atmosphere trapped in ice: 800,000 years of climate change"
16 November "New hydrocarbon development challenges and the impact on geoscience research"
14 December "Mineral deposits and their global strategic supply"

Reserve a seat by contacting The Geological Society
Burlington House, Piccadilly, London W1J 0BG
Tel +44 (0)20-7434-9944
Email registration@geolsoc.org.uk

More information about The Geological Society is available at:
www.geolsoc.org.uk/shelllondonlectures11