The geology of the San Francisco area is complex, and is related to tectonic events over the last 200M years. Today, the North American plate lies next to the Pacific plate. 200M years ago, however, there was another plate composed of oceanic crust lying between these two – the Farallon Plate – named after the Farallon Islands which lie off the West Coast of the USA and are on a small remnant of the Farallon plate. Most of the Farallon plate has now disappeared, having been subducted under the North American plate.

The Juan de Fuca micro-plate and the Rivera micro-plate are surviving remnants of the original Farallon plate.
30M years ago, the subduction of the Farallon plate was finishing, and the Pacific plate was coming into direct contact the North American plate. At this time, the subduction process ended, and the Pacific plate started sliding Northwards along the edge of the North American plate. This transform faulting created the San Andreas Fault system which extends for hundreds of miles up and down the West coast of the USA. Movement along the San Andreas fault created the San Francisco earthquake in 1906.

Point Reyes is a promontory projecting West into the Pacific Ocean about 30 miles North of San Francisco. The relief map above shows the line of the San Andreas fault very clearly as a diagonal line heading roughly NNW – SSE. The line of the fault is marked by inlets of the sea such as the Tomales Bay. Between these inlets, the fault line is low-lying, often marshy land.
All the land West of the fault line lies on the Pacific plate. Careful correlation of the rock formations on both sides of the fault has shown that the Pacific plate has moved hundreds of miles North relative to the North American plate since the formation of the San Andreas Fault. The rocks which make up the actual point – the Point Reyes Conglomerate – match an identical area of rock near the Monterey Peninsula, 100 miles to the South.

While the Farallon plate was subducting, several episodes of accretionary prism formation were occurring. Rock is accreted against the American plate instead of being subducted.
The oldest prism is the highest in the sequence – the opposite of what is seen in sedimentary beds. Between the prisms, there is concentrated movement with grinding of rock and subsequent melange formation.

San Francisco Bedrock Today

There are five distinct layers underlying San Francisco in a sequence from East to West:
Alcatraz Sandstone
Hunters Point melange
Marin Headlands terrane
City College melange
San Bruno Mountain Sandstone
The two melanges are similar and were caused by intense shearing during the formation and subsequent exhumation of the accretionary prisms. They comprise a mixture of rock types and size ranging from large boulders hundreds of metres across to fragments <<1mm. The small fragments are easily eroded, leaving the boulders behind. This is commonly seen on the sea shore. Also, landslides and slumps are common within the areas of melange.

Large blocks of serpentinite in the Hunters Point Mélange. (my rucksack for scale)

The sandstone areas look similar to the naked eye, but they are very different geochemically. The Marin Headlands terrane is a patchwork of different rock types in close proximity, pillow basalt, red chert and sandstone – but not a melange.

Alcatraz Island out in San Francisco Bay is composed of thick beds of Alcatraz Sandstone

The Marin Headlands terrane containsvarious rock types including pillow lavas. A4 folder for scale

Angel Island
Further out in San Francisco Bay than Alcatraz is Angel Island. This was a military site, but nowadays it is a nature reserve and tourist attraction. Geologically, it is very interesting because most of the rocks are metamorphosed. This points to the fact that the rocks on Angel Island were buried deeper in the subduction zone than the other rocks in and around San Francisco.
The original sandstone, pillow basalt and serpentinite have become schistose sandstone, schists, blueschists, metamorphosed pillow basalt, and metamorphosed serpentine. The blueschist in particular, is taken to be a marker of subduction zones.

Schistose sandstone pebbles incorporated in the stone can still be seen but the outlines are blurred as a result of the metamorphosis

Bluschist within a beach exposure on Angel Island

Metamorphosed pillow lavas

Short History of San Francisco

San Francisco developed rapidly as a port and urban centre following the California gold rush in 1849. Following extensive damage in the 1906 earthquake, the city was rebuilt in time to host the 1915 World Fair, which established the city as a significant world trade centre.
Nowadays, the city is better known for hippie culture and lifestyle, for tourism and for the adjacent silicon valley enterprises.

Lake at the Palace of Fine Arts

Sea Lions in the Fisherman's Wharf area on the northern edge of the city

July Evening Field Trip

On Wednesday 18th July ten members enjoyed our “traditional” evening field trip which we take in July to enjoy the long evenings out of doors. The weather was glorious as we explored the classic Aberedw Rocks location, along the lines of the “Geological Excursions in Powys” Guide, edited by N.H. Woodcock and M.G. Bassett (1993) pp301-307. This is Murchison country, where he established the sequence down from the unfossiliferous Old Red Sandstone to his fossiliferous Silurian System.

Group discussing seqence at first stop

The Silurian rocks we were examining were Late Ludlow, (Ludfordian stage), just off the shelf edge. We would work up the succession, starting in Aberedw, where we met in the car park of the local pub (Courtesy of the Seven Stars). The first location was a few yards below, on the river bank where there were low cliffs. The beds exposed were very thinly (~mm) laminated dark grey shaley siltstones There were a few burrows and very few fossils (one disputed Lingula?) These were the Upper Lingula lata beds and it could be autosuggestion!

Dark grey siltstones of the Upper Lingula lata beds in river cliff

They were cut by a fault (exploited interestingly by a tree root) and, in the beds above in the road cutting there were small ribbed brachiopods. Maybe the fault had brought down higher beds.

Tree root exploiting fault

In the river bed the generally southerly dip of ~30deg was evident.

Southerly dipping beds in river

We condensed cars and moved on to the second location at Pont Shoni, less than a kilometre to the south where (thanks to the owners) we parked in lovely gardens which exploited the natural pillar-like thin bedded (~20-50mm) near horizontal dipping flaggy siltstones.

Progressing through the gardens we joined a little used footpath which took us up a significant gorge in gently dipping paler flaggy siltstones which became more fossiliferous as we ascended the scree in the gorge. (Thanks to Colin and Tony for beating a path through the nettles on the previous ‘recce’). We were up sequence and in the Transition beds, equivalent to the Upper Leintwardine and Lower Whitcliffe Formations at Ludlow. As we ascended, we found more and more fossils including brachiopods, nautiloids and a gastropod.

Fossiliferous scree in the gorge

Ascending the gorge

As we worked up the sequence we could see how the gradual sea level regression taking place was enabling more shelly shelf-derived sediments to come in from the east.

Looking at the view from the top, we appreciated that other locations nearby would warrant a whole day excursion at some future occasion, however we returned to the cars having made good use of a lovely evening.

Tony had been to the isle of Arran in the Clyde estuary twice. First as a student in 1954 and then last year with Joe Botting’s palaeontological group. He remembered precious little of the first trip, apart from the ferry, which made an impression and, once on the island, sheltering from a blizzard somewhere up Goatfell.

That was before the days of “Ro-Ro”. In ‘54 a more modest ferry went from Fairlie to Brodick and comprised a small steamer which sported a polished brass plate stating that she served as HMS Goatfell during the war, firstly as a minesweeper and then, with guns bolted to the deck, as an ack-ack vessel. It must have been choppy when they crossed as he remembered a pig in a crate on deck had to be retrieved. One could get close and personal to the rather beautiful triple expansion steam engine too. It is a shame that such a vessel met an ignominious end. Sold to Bass-Charrington, she served as a floating pub tied up to the Victoria Embankment until she caught fire in 1980 and was finally scrapped. Her engines were rescued and are at a museum near Liphook, but not on display.

Arran exemplifies Scottish geology in miniature, igneous, metamorphic, sedimentary and both “Highland” and “Midland Valley” types. These were investigated with Joe during a glorious week last year. However, there are many “Geo Tours” of Arran available in the literature and on the web, so Tony expanded on items which made a special impact.

Hutton, and his Unconformity
Walking round the north east coast, at the most northerly point, the group encountered “Hutton’s Unconformity”. There are a number of “Hutton’s Unconformities”, but this was the first one he found. A better known one is at Siccar Point, and another famous one is near Jedborough.

Hutton's unconformity

They are mentioned in the first chapter of every geology text book and represent a key moment in our understanding of the world we live in, when the literal biblical model of how our planet got to be how it is was shown to be untenable. Hutton realised that an angular unconformity, where bedded sedimentary rocks, all presumably laid down horizontally, were overlain by beds at a different angle must represent a very long time interval during which normal processes of erosion, deposition and vertical movement occurred. He founded the “uniformitarian” movement.

It is quite astonishing that, at this very significant location, there is nothing to indicate how special it is, no sign, no information board, no monument! Even passing locals were unaware of the location and indeed, it took a while to locate it precisely.

Looking for the unconformity

The unconformity

But, what of Hutton himself, and how is it that, outside of a specialised discipline, he is so unrecognised?
He was born in Edinburgh, the son of a merchant who died when he was 3 years old. At High School he excelled at maths and chemistry but on leaving, he was apprenticed to a lawyer. He preferred chemistry and unsurprisingly, was sacked at the age of 18. He then became a physician’s assistant, attended lectures at the University and, further studying, went on to Paris before graduating MD from Leiden.

Returning to Edinburgh in 1750, he teamed up with schoolfriend James Davy to start a successful factory making sal ammoniac from soot.
He took seriously the management of a family farm in Berwickshire, going to Norfolk and Holland to study best methods. He was not averse to getting mud on his boots and could not resist sticking his nose into every ditch and quarry. Presumably this paid off as he was eventually able to afford a manager.

It was the time of The Enlightenment, when Edinburgh was at its zenith and he was rubbing shoulders with John Playfair, the mathematician, Joseph Black, the chemist and Adam Smith, the economist. In 1783 he read a paper to the Royal Society of Edinburgh on his “ Theory of Earth.” in which, contra to biblical teaching, he proposed that:
1 Land was not original, but composed of products of earlier lands. (Like found on the shoreline.)
2 Before the present there had been earlier lands with tides and currents like at present.
3 The sea had been inhabited by animals as at present.
So the land had been produced by processes which operated as at present.
For this to occur, two things were required:

1. Means for sediment to consolidate into rock.
2. Means of altering elevation

Later he went on a tour with John Clerk to confirm his theory, visiting Glen Tilt, where he noted granite penetrated the “schistus” and cooked the contact showing that it cooled from molten and was not precipitated from water as Werner’s Neptunian/catastrophic theory postulated. Similar effects were noted at “Hutton’s Section” in Edinburgh. That and his various unconformities all confirmed his theories. His “Theory of the Earth, with Proofs and Illustrations” was published in 1795.

This made little impact, possibly because his language was somewhat tedious and convoluted; however his friend, John Playfair later wrote “Illustrations of the Huttonian theory of the Earth” (1802) which proved popular and was taken up by Charles Lyell in his “Principles of Geology”.

Hutton and Darwin
As such, Hutton significantly influenced Darwin and it is worth noting that Darwin took both these books on board Beagle.
Hutton also advocated uniformitarianism for living creatures – evolution, in a sense – and even suggested natural selection as a possible mechanism affecting them. Quoting directly from “Investigation of the Principles of Knowledge, volume 2.”
"...if an organised body is not in the situation and circumstances best adapted to its sustenance and propagation, then, in conceiving an indefinite variety among the individuals of that species, we must be assured, that, on the one hand, those which depart most from the best adapted constitution, will be the most liable to perish, while, on the other hand, those organised bodies, which most approach to the best constitution for the present circumstances, will be best adapted to continue, in preserving themselves and multiplying the individuals of their race."

Hutton gave the example that where dogs survived through "swiftness of foot and quickness of sight... the most defective in respect of those necessary qualities, would be the most subject to perish, and that those who employed them in greatest perfection... would be those who would remain, to preserve themselves, and to continue the race"

Bearing in mind that was published in 1794 and the “Origin of Species” in 1859 how is it that, outside his subject, his unconformities and his ‘section’, Hutton is memorialised by one garden, one Institute (since 2011) and one road (James Hutton Road since 2016); whereas Darwin has innumerable roads, colleges, universities, towns and even a city to his name!
It may be because Hutton was neither an aristocrat nor an academic. We only have one indifferent portrait of him. in which he appears rather po-faced, not the man with mud on his boots with his nose in a ditch! Not being an academic, he was casual about his publications, he actually lost 70 illustrations for his final masterpiece which only turned up in the 1960s

Hutton

The Corriegills Pitchstone
Tony has been intrigued by pitchstones for some time because they are very rare, but several varieties occur among the dykes and sills in the British Tertiary Volcanic Province. In thin section, they are quite unlike any other rock; so last year he stayed on an extra day to visit the Corriegills location on the coast a few miles south of Brodick.
The exposure did not disappoint, it is in the cliff, above high water mark and lookes like black marble

The exposure
Similar to obsidian, pitchstone is less glassy, has a hackly fracture and contains more water. It has a granitic chemical composition and in thin section is glassy with masses of feathery growths resembling little fir trees of the order 100 micron long.

Thin Section

There are also clouds of very small microlites showing some flow structure which are depleted near the growths. The suggestion is that the larger growths are hornblende but the microlites are indeterminate. Pitchstones contain 5 to 10% water, far more than obsidian and this has profound effects. Water has the ability to break the polymer chain in silicates and reduce viscosity. It is thought pitchstones could originate by contamination of basaltic magmas with hydrous crustal material thus producing a low viscosity high silica melt which solidifies to a glass if cooled quickly as a dyke or sill.
Pitchstone is a rock the structure and derivation of which are still in debate.

Due to unforseen circumstances Dr John Davies will not give the talk on the 15th August. Instead Dr Chris simpson will give a talk entitled: "An Overview of San Francisco Geology."
The talk will summarise the last 200M years of geology as it affects the West Coast of the USA, and how this can be seen in the rocks of the San Francisco area today. It will also include some historical aspects of San Francisco, and a trip to the Yosemite National Park.

At the last meeting we had a complete change from geology. Dr. Richard Carter gave us a brief insight to his field of work covering interactions between humans and the environment during the Mesolithic Period.

In order to understand human behaviour at that time it is necessary to understand the environment in which they found themselves. Humans colonised Britain hundreds of thousands of years ago but displacement and recolonisation occurred related to major glaciations and fluctuating ice-sheets with the final (Devensian) Ice-Age ending about 10,500 years ago.

During the early Mesolithic warming temperatures led to the melting of continental glaciers and a rise in sea level. This caused deltas and coastlines to be inundated and low lying land to be flooded. The warmer climate also encouraged a change in vegetation with the development of pre-boreal forests of pine, birch and alder. This was an open landscape. Over time the pre-boreal forest was replaced by boreal type consisting of more deciduous species like hazel, oak and lime with the pines being forced to higher and drier areas. Dense forests with occasional breaks now occurred. By the time of the late Mesolithic temperatures were equivalent to what we experience today and the forests were dense. Throughout this time the changing environment meant changes in the flora and fauna which formed the diet of Mesolithic peoples. This in turn led to the development of new methods and tools for hunting such as the microliths for fixing onto harpoons and spears.

Throughout the Mesolithic the rising sea levels eventually saw the land-bridge which connected Britain to the rest of Europe disappear and Britain became the island it is today.

Dr. Carter also gave a short explanation of modelling hunter-gatherer mobility as this can allow inferences about occupation patterns and types of sites that are used. It may also help to distinguish between long and short term occupation.
This was a fascinating and informative talk.

The next meeting will be held on Wednesday June 20th when member Tony Thorp will give a talk entitled, “ The Geology of Arran”.