Concretions in sandstones of the Inner Hebrides, Scotland

Mark Wilkinson (UK) Concretions are a common feature in many sedimentary rocks, yet they seem sometimes to be misunderstood. So, how do concretions form? As well-studied examples, let’s look at the ones found in some of the sandstones of the Scottish Inner Hebrides, notably the islands of Eigg and Skye. The concretions are found in several formations, but perhaps the largest and most spectacular are in the Valtos Sandstone Formation of the Great Estuarine Group. This was originally named the Concretionary Sandstone Series after the prominent metre-scale concretions. It is Bathonian in age (Middle Jurassic) and is interpreted as having been deposited in a coastal environment. The Great Estuarine Group is becoming famous for its abundant dinosaur footprints and much rarer skeletal material. The concretions themselves vary from spherical to elongate volumes of rock and are typically from around 50cm to one metre or more in diameter. They are also often coalesced into groups (Fig. 1). Inside the concretions, the spaces between the sand grains are filled completely with a calcite cement. The concretions are resistant to weathering compared to the host sandstone, which is fairly soft, so stick out from the cliff in a sometimes rather alarming manner as you walk below them. I’ve been visiting the concretions sporadically for around 30 years and some of the ones that I photographed in the cliffs in the 1980s are now lying loose on the beach. None of them have fallen while I’ve been there, touch wood. Fig 1. Concretions on … Read More

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Rudists: A fossil story

Jack Shimon (USA) This article is adapted from a presentation given at the Denver Gem Show, September 17, 2016 by me, Jack Shimon. When I was six and a half years old, my Grandpa took me fossil hunting in central Texas. We went to a Carboniferous Limestone quarry that he had visited earlier and was given permission to enter and collect from. This was one of my first fossil hunting trips and I really enjoyed it. The ancient reef we went to (now a quarry) had huge boulders of limestone and tube-like things in it we later to be found to be rudist bivalves. This article is all about these finds and the efforts we went to, to find out what they were. Fig 1. The author at the quarry. (Photo credit: Mike Hursey.) Fig. 2. This Google satellite image shows the reef we collected from. Two of the three lobes have been excavated for limestone. You can also see smaller pinnacle reefs marked with the short arrows. All of the reefs rise above the flat Texas landscape. (Permission from Google.com: ‘Special Use Guidelines’.)Fossils We spent a lot of time at the quarry observing the massive specimens onsite and then collected some smaller pieces to bring home and look at closer. A simple way of thinking about fossils is to consider them either as a cast or a mould. A mould is formed when an object is placed into a soft substrate and then decomposes or is washed away leaving … Read More

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Rocks in Roslin Glen: A record of a swampy past

Mark Wilkinson and Claire Jellema (UK) Midlothian is an area of central Scotland that lies to the west of Edinburgh and is an area with strong geological connections due to a history of mining for both coal and oil shale. As a part of the annual Midlothian Science Festival (http://midlothiansciencefestival.com/), the School of GeoSciences at the University of Edinburgh offered a walk to look at some local geology and a talk about climate change research on the Greenland icecap. In addition, a ‘Dino and Rocks Day’ was attended by 380 people, proof (as if it were needed) that dinosaurs continue to fascinate the general public. The Edinburgh Geological Society also contributed with a session about Midlothian Fossils and a local historian talked about the history of coal mining in the area. The geology walk visited local exposures, in this case Carboniferous sediments including what may be the best exposed fluvial sediments in the area. The walk was advertised as “Rocks in Roslin Glen: a Record of a Swampy Past” and all 25 spaces were quickly booked. The location was Roslin Glen, which may sound familiar if you’ve seen the film, The Da Vinci Code, based on the novel by Dan Brown. We have not misspelled the name of the glen incidentally. For some reason, Rosslyn Chapel lies on the edge of Roslin Glen and the country park of the same spelling. The glen itself is a steep-sided valley of around 20m in depth, which carries the River North Esk roughly … Read More

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What’s so special about South Devon?

Professor John CW Cope (National Museum of Wales, Cardiff UK) Take a trip to the South Devon coast around Easter time and you are bound to come across student parties from universities engaged in fieldwork. What is it about this area that makes it so popular as a centre for this? The simple answer lies in a single word — variety. There is probably no other area in the UK where such a wide variety of rock types and ages is well-exposed in such a small geographical compass. Let’s have a look at some of the factors. The geological succession The oldest rocks exposed in South Devon are of Devonian age and, unlike many other areas of the UK, the Devonian rocks are in marine facies virtually throughout. Looking back over the history of geology, the age of these rocks had initially proved difficult to identify and it was only after Murchison had seen the marine successions in The Rhineland and Russia that he realised that these marine rocks were the equivalent of the Old Red Sandstone farther to the north. The Devonian rocks present a variety of marine facies, with the Middle Devonian limestones being of particular note. The limestones are a local development whose presence, in an otherwise deeper water succession, is due entirely to local shallowing of the water caused by thicknesses of volcanic rocks extruded along extensional fault lines as the local basins developed. This shallowing allowed reef-building organisms to flourish and the principal ones of … Read More

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Carboniferous fossils protecting the coastline at Barton on Sea

David N Lewis (UK) and Stephen K Donovan (The Netherlands) Many people regard fossils, quite rightly, as rare and exotic objects. Yet how often do people come into contact with palaeontological remains without appreciating it? Probably the easiest example to cite is that of quarried stone, either appearing as facing stones or, in a less aesthetically pleasing setting, when ground down or crushed for concrete or road ballast. Often, quarried stone is utilised a large distance from its source. For example there are no exposures of Carboniferous Limestone in the Netherlands, yet this rock is common in Dutch towns and cities where it is found as facing and decorative stones, far from its origins in Belgium and elsewhere. Obviously such uses of rock are to be admired visually but not hammered; yet this is not necessarily always the case. In this article we introduce you to exotic blocks of Carboniferous Limestone which are so situated that they are actively worn down by the elements, exposing the treasures contained within. Fig. 1. Maps of southern Britain and Christchurch Bay (after Lewis et al. 2003). The cliffs of the famous fossil collecting area of Barton on Sea are part of the (often slumped) sea cliffs of Christchurch Bay in Hampshire and Dorset, extending, in the west, from Friars Cliff, near Christchurch, to Milford-on-Sea, near Lymington in the east (Fig.1). These are composed of Eocene clays and sandstones, overlain by Pleistocene plateau gravels (Fig. 2) and have been systematically eroded over long periods … Read More

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Arthropleura: A prehistoric bug hunt

Joe Shimmin and Stephen Day (UK) Picture yourself strolling through lush, green woodland, on an Earth unspoiled by man and yet to witness the rise of the dinosaurs. You’d be forgiven for feeling at peace with the world, even slightly euphoric – that is until you stumbled across the giant Arthropleura, a millipede relation as long as a park bench. This encounter might make even the most enthusiastic creepy-crawly hater think twice before squashing the bug in front of them under foot! I (JS) had a slightly less dramatic (but still very exciting) experience involving the creature while on a recent fossil hunting trip to Crail in Fife. On investigating some sandstone ledges that ran across the shore to the south-west of this pretty little fishing village in western Scotland, my eyes were drawn to what could only be a huge set of fossil tracks in the rock. The stratum in which they had been preserved also contained plant remains such as Stigmaria roots, as well as sections of tree trunks and branches. Fig. 1. The pretty fishing village of Crail, Fife as seen from the Arthropleura track find site. I took numerous photographs of the track, which measured about 3m long by 30cm wide and also of other, similar tracks nearby, in the hope that someone might be able to identify what kind of creature had created them. My guess was that it was some sort of amphibian, but I wasn’t sure. All I knew was the thing that … Read More

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Geology at Loch Lomond and the Trossachs National Park

Ruth Crosbie (UK) The Loch Lomond and the Trossachs National Park has a unique and very visible geological character. This, and the geomorphological processes that have taken place in the area have been fundamental in shaping the outstanding landscape and scenery of the park. Fig. 1. The outstaniing landscape and scenery, seen today at Lock Lomond and the Trossachs National Park, has been shaped over millions of years by geomorphological processes. Rolling, relatively low-lying farmland along the southern margins of the park is underlain by Silurian to Carboniferous sedimentary rocks. North of the Highland Boundary Fault, this rolling country gives way to increasingly mountainous land, underlain by more ancient metamorphosed rocks. Many of the visible landforms represent the actions of glacial processes. Classic ‘U’-shaped valleys, such as the north Loch Lomond basin and Strathfillan, were carved by glacial ice. Other features, such as drumlins near Tyndrum and the rolling landscapes south of the Highland Boundary Fault, are the result of sediments deposited by melting glaciers. Such contrasts in the geology and landforms are reflected in similar marked contrasts in land-use patterns. Geological Structure The park contains a wealth of geological and geomorphological features, including some of national and international importance. The Highland Boundary Fault, which separates the Highlands from the Scottish Midland Valley, is well known. Within the park, the fault runs from Arden through Balmaha, Aberfoyle and Loch Venachar, and its line is clearly visible through the islands of southern Loch Lomond. Although less well known, other features include … Read More

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Geology and terrestrial life of the Carboniferous

Russell Garwood and Alan Spencer (UK) The Carboniferous Period is a fascinating time in earth history. It spanned 60Ma (359.2 to 299.0Ma), towards the end of the Palaeozoic era, falling between the Devonian and Permian. During the Carboniferous, the supercontinent Pangaea was assembling and the oceans were home to invertebrates such as corals, bryozoa, ammonoids, echinoderms, trilobites and crustaceans. Fish were also well represented (especially sharks), which were rapidly diversifying at the time. The continents were no barren wasteland either – they were host to some of the first widespread terrestrial forest and swamp ecosystems. In these lived both invertebrates, which had crawled onto land by the Silurian period (at least 423mya) and vertebrates, which were relative newcomers to this realm. This article provides us with an excuse to write about the Carboniferous. We will first introduce the geology and palaeogeography of the Carboniferous, including an overview of the most common mode of preservation we see in terrestrial fossils. Then, we will provide an overview of terrestrial life during the period, as land-based ecosystems of this age are among the best known from the Palaeozoic and an exciting time in the history of life. Fig. 1. Global paleogeographic reconstruction of the Earth in the late Carboniferous period 300mya. (C)opyright Dr Ron Blakey (Wikipedia Creative Commons). Carboniferous geology The Carboniferous is split into two epochs, the Mississippian (or Lower Carboniferous; 359.2 to 318.1mya) and the Pennsylvanian (or Upper Carboniferous; 318.1 to 299.0mya). As we shall see, the two are associated with … Read More

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