Gravel sheets in the suburbs of Washington, DC

Deborah Painter (USA) If you live in western Prince George’s County, Maryland in the USA, in the towns of Oxon Hill and Suitland and you want to dig to place a water line, plant a garden or excavate to construct a foundation for any building, chances are you will encounter sandy soil with hundreds of cobbles and boulders. Some boulders encountered could be in the form of large flattened slabs. You might be wondering why these are present, since these towns are in a coastal plain, far south and east of the rocky outcrops of the Piedmont area of Virginia and Maryland. For someone like me, who was born and raised in the Coastal Plain area of Virginia, these ubiquitous cobbles and boulders seemed out of character for the region. I discovered these odd boulders and cobbles when I joined a colleague from an office in a northern state to assist him in ecological studies for two small sites not too far from the United States Capital of Washington, in the District of Columbia (DC). Our goal was to help our client know if there were any threatened or endangered species, wetlands, hazardous materials or other site constraints, as this would assist the client to decide whether to purchase the properties. Our first Prince George’s County site for an ecological study was one of a few hectares in size in Suitland, a suburb of Washington, DC and approximately 8km southeast of the border of the capital city near the shore … Read More

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Colourful bluffs in Long Island recall the most recent ice age

Deborah Painter (USA) Imagine a tremendous piece of land moving equipment that scraped up the soil and some of the surface bedrock from four states within the United States’ Eastern Seaboard, carrying and dragging it all the way, before dumping it on a ridge off the shoreline. That is what essentially occurred with the final advance of the Wisconsinian ice sheet, the only one which left glacial deposits visible in New York State today. Long Island is a ridge of Cretaceous bedrock with glacial deposition. The moraines there have not been ground into sandbars and spits along the western end of the north shore as much as elsewhere, because of the sheltered nature of the Long Island Sound. Therefore, shoreline bluffs expose rocks as well as glacial loess. Fig. 1. Fishermen’s Drive takes you to the loess deposits. To park at the beach requires a permit. (Photo by JB Steadman.) If you find that your journeys take you to New York City, one of the world’s largest metropolitan areas, try to make time to visit Caumsett State Park at Long Island Sound. My own visit began when planning a visit to New York State’s Long Island to see my friend, Joyce Raber. She suggested various things that we might do: go to a Broadway play, go shopping and so forth. However, my list of things to do was typically “eco-tourist”. I wanted to visit the famed American Museum of Natural History in Manhattan, then see nearby Central Park, where the … Read More

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La Gomera: A short geological guide

Ken Madrell (UK) The island of La Gomera has an area of 370km2, it is 25km in diameter, has a maximum altitude of 1,487m (Alto Garajonay) and is situated approximately 40km west of Tenerife. Unlike the other Canary Islands, La Gomera has experienced a long and continuing eruptive break and is in a ‘postshield erosional stage’. Carracedo and Troll (2016) describe this as the stage when active volcanism has ceased, and erosive and denudational landforms are predominant (p. 39). The submarine base of the island shows that it rests on a shallower ocean bed than the surrounding islands. The emerged land mass is semi-circular in shape, with a radial drainage pattern from its centre near Alto de Garajonay. The dating of the island has proved problematic, as some of the earlier measurements placing its age between 15 Ma and 19 Ma have since proved to be inaccurate. More reliable estimates now put its age at between 10 and 11 Ma. Fig. 1. Roque Argando viewed from Lomo de la Mulata. La Gomera’s general stratigraphy comprises of three main rock sequences: (1) A Miocene basaltic shield, including a basal plutonic complex (that is igneous rock formed by solidification at considerable depth beneath the earth’s surface); (2) A nested felsic (that is, igneous rocks that are relatively rich in elements that form feldspar and quartz) stratovolcano (which is built up of alternating layers of lava and ash); and (3) The youngest Pliocene volcanism. Fig. 2. Sketch map of La Gomera, showing the … Read More

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Armboth Dyke, Lake District

Mark Wilkinson (UK) The Armboth Dyke makes a good half day geology excursion in a scenic but quiet part of the UK Lake District. Parking is on the west shore of Thirlmere, in a pay-and-display car park accessed by the narrow road that winds around that side of the lake (Grid reference NY 305 172). The car park is in an excellent setting, with direct access to the wooded lake shore, and would be a great place for the non-geologically minded to wait while you venture onto the adjacent hill. It is probably worth noting at this point that the dyke itself is mostly exposed on rather featureless rolling moorland at around 400m above sea level (Fig. 1), and might not be a good place to visit in thick mist, unless you are very confident with a map and compass. If you happen to be in the business of teaching students to make geological maps, this site makes a great practise day, without too many problems of recognising weathered rocks in the field. Fig 1. Moorland with the dyke just visible as a slightly lighter patch of rock below the red arrow, where the edge of the dyke is exposed. University of Edinburgh students for scale. So, assuming you have decent weather, leave the car park and take the path uphill from the west side of the minor road, a few metres to the north of the car park. The path is steep-ish, and can be slippery if wet, so … Read More

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Sand for arenophiles

 Dawn Walker (UK) As long as I can remember, I have collected interesting bits of rock, looking at their shapes and colours, and wondering what they were. This was fine as long as I had a garden shed of my own to keep them all in. I read some geology as a hobby and began to recognise a few of them, but then age caught up with me and I had to move to a smaller flat and there is simply no room for more rocks. In fact, I had to dispose of many of my old ones. Eventually, it dawned on me that sand is also rock, although made up of rather small pieces, and would not take up too much room, so why not collect that? After all, I was living at the seaside. Fig. 1. These translucent green grains from South Point, Hawaii, are olivine. The pounding surf erodes a forty-nine-thousandyear- old volcanic cinder cone made of olivine. As the cone erodes, the olivine crystals become beach sand. Very few beaches consist of pure olivine; however, if there is olivine in a sand, that indicates it is a volcanic region. Magnified 250 times. I really thought I had invented this hobby and was amazed to discover on the Internet that I was an arenologist or possibly an arenophile (from the Latin arena, meaning sand). There is an International Sand Collectors Society in America, which I joined, and now have email friends all over the place, with whom … Read More

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Inclusions in precious and semi-precious gemstones

Dr Robert Sturm (Austria) Gemstones are commonly regarded as natural wonders, and their infatuating beauty and rareness has fascinated us from time immemorial. Besides the obvious macroscopic appearance, many a gemstone is characterised by a ‘hidden’ microscopic inner life of breathtaking aesthetics. Among non-experts, such inclusions in precious and semi-precious gemstones are often interpreted as ‘pollution’ or as ‘blots’. On the other hand, among gemologists, inclusions bear valuable information about the genesis of their hosts and may also increase the value of a stone. The main characteristics of inclusions in precious and semi-precious gemstones Basically, inclusions in gemstones occur in three aggregate states: solid, liquid and gaseous. Solid inclusions are generally represented by those minerals found in close vicinity to the host stone or correspond with the chemistry of the host stone. These mineral inclusions either crystallise before their host (protogenetic), at the same time (syngenetic) or after its formation (epigenetic). Epigenetic crystallisation of inclusions takes place in most cases by so-called dismixture processes during the cooling of the host stone. Inclusions being generated in such a way are commonly characterised by the same orientation as the host crystal (for example, needles of rutile in corundum – rutile is a mineral composed primarily of titanium dioxide). Liquid and gaseous inclusions are often marked by some kind of coexistence, so that they are summarised by the term “fluid inclusions”. They have to be regarded as a consequence of the fact that many gemstones form from a liquid or aqueous medium, and … Read More

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Mineral classics from Wales

Tom Cotterell (UK) Ask any mineral collector to name a classic mineral locality or region in Britain and they will probably think of Cornwall or Devon, perhaps Weardale in Co Durham, or even the Caldbeck Fells or the West Cumbrian iron mining district in Cumbria – but probably not Wales. This is not to say that Wales has no classic minerals, but is perhaps a reflection of collecting habits and the preference for large, brightly coloured crystals. Wales has a long history of mining dating back to, at least, the Bronze Age, but, unlike some other regions, there does not appear to have been a desire by miners to extract mineral specimens for sale. Indeed, a network of mineral dealers, as was clearly present in Cornwall during the nineteenth and twentieth centuries, was totally absent in Wales. One factor is that the establishment of a National Museum in Wales occurred relatively late (in 1907) and did not open to the general public until the 1920s. Before this, there was no central repository for specimens collected in Wales and, consequently, mineral collections with historical significance are rare in the Principality. The university colleges founded during the 1870s and 1880s built up their own academic collections. Earlier still, the Royal Institute of South Wales (founded in Swansea in 1835), established geological collections, but its focus appears (from what records remain) to have been wide ranging and not specific to Wales. Therefore, during the heyday of mining in Wales, the lack of one … Read More

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Natural wonders of the Maghreb in Morocco

Sebastian Lüning (UK) Morocco is a popular tourist destination. Most people travel to the white beaches of Agadir to sunbathe and relax, to watch the magicians on Djemaa el-Fna square in Marrakech, or to go shopping in the UNESCO-protected Osouk of Fes. However, Morocco has much more to offer. Some of the most attractive specimens found at international fossil fairs originate from this country. Morocco is home to exceptionally well-preserved trilobites and attractive Orthoceras assemblages from the Palaeozoic. The beds containing these fossils are systematically mined in the Anti-Atlas. Other fossils, such as goniatites and ammonites, complement the diverse palaeontological national treasure. Fig. 1. Location map of geological sites mentioned in this article. 1) granites near Tafraoute, 2) algal mats near Ouarzazate, 3) Ordovician glaciation, 4) Silurian graptolithic shales, 5) Orthoceras limestones, 6) Devonian mud mounds and Merzouga sand dunes, 7) Triassic Argana river sands, 8) Cascades d’Ouzoud, 9) Friouato karst shaft, 10) Dades Gorge, 11) blowholes near Agadir and Cretaceous oysters, 13) Amesfrane cliff. These fossils are part of an exciting geological past. This article aims to guide you through the highlights of Morocco’s geological history, exploring the stories behind the country’s natural wonders and its multi-million-year-old inhabitants. Concealed in its spectacular mountain chains are some fascinating snapshots from the past. Our trip will commence at the very beginning of this history and will take us gradually forward through time. We will visit various sites on a route starting in the Precambrian of the Anti-Atlas, in the southern part … Read More

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Mineral collecting in Slovakia

Trevor Devon (UK) Slovakia is situated at the north-western end of the Carpathian Mountains, a region well-known for its metal ore mines and quarries. One of the Sussex Mineralogical Society’s members had been a schoolteacher in Slovakia and had explored many of its mineral locations. Through his contacts there, an 11 day visit was arranged and a quite large group, comprising 17 society members, descended on the rural tranquillity of eastern Slovakia in August 2008. We were met by our two expert guides, one of whom was Dr Rudolf Ďud’a, head of the Department of Natural History in the Eastern Slovak Museum in Košice and author of the Slovakia chapter in the book Minerals of the Carpathians. Our journey took us on a round trip from eastern Slovakia up to Prešov and the Tatra Mountains on the border with Poland, across to Banská Bystrica in central Slovakia, south to šiatorská Bukovinka near the Hungarian border and back to Košice. Much of the driving was through heavily wooded mountains and attractive scenic valleys. The mines (now mostly inactive) and quarries were often well hidden, and generally required some walking (always upwards, of course) to get to from our coach. During our trip, we visited four quarries, eight mine dumps and a wooded mountainside deposit of ‘flesh opals’, so we were kept busy. One of the lasting impressions of the mine dumps (some of which were very large indeed) was the richness of secondary mineralisation – when in Cornwall, one is usually … Read More

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Rose and blue quartz

Steven Wade Veatch (USA) Quartz (SiO2) is a common mineral found in all three classes of rocks (igneous, metamorphic and sedimentary), in many environments and in a range of colours. However, rose and blue quartz are less common than some of the other varieties. This article discussed these two extraordinary minerals. Rose quartz Rose quartz has a pale pink to rose-red colour, thought to be caused by trace amounts of titanium, which absorbs all colours except pink. In a laboratory experiment, samples of rose quartz from several localities were carefully dissolved in acid. The remaining insoluble residue consisted of thin microscopic fibres, which may also be responsible for the colour of rose quartz. Well-formed rose quartz crystals are rarely found in nature, but when they are, they are generally found in massive chunks associated with pegmatites (Fig. 1). Fig. 1. This large rose quartz specimen was found at the Devil’s Hole Mine (owned by Tezaks), about a mile from the town of Cotopaxi, Colorado. (Photo © 2007 A Schaak.) The term pegmatite refers to exceptionally coarse-grained crystalline granite. Since rose quartz is cloudy, it is not popular as a faceted gem, but it is commonly made into cabochons (Fig. 2), rounded into beads for necklaces or carved into various objects. Fig, 2. A cabochon pendant from the same rose quartz near Cotopaxi. (Photo © 2007 A Schaak.) It has been named as South Dakota’s official state mineral. Here, rock hounds have a good chance of finding specimens ranging from shades … Read More

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On the origin of agate: A 300-year-old enigma

Terry Moxon (UK) Quartz has been estimated to occupy around 12% of the earth’s crust and can be found in many forms, ranging from the massive, clear crystals of quartz and amethyst to the microcrystalline quartz that is to be found in jasper, agate, chalcedony, chert and flint. World-wide, the distribution of agate is not equal, but it can be found in every continent and probably exists in every country. However, only three countries extract enough agate for world export: Botswana, Brazil and Mexico. Fig. 1. Empty gas cavities and agate amygdales in a block of Isle of Mull basalt. (Scale bar = 2cm.) Agate is most frequently found in fine-grained, igneous rocks filling gas cavities (Fig. 1), but it can also be found in sedimentary limestone hosts (Fig. 2) and fossil wood (Fig. 3). The most common agates are the wall-lining and horizontally banded types (Figs. 4 and 5 respectively). Rapid identification of agate in the field relies on the natural translucency of a fractured sample, but final confirmation is supplied by examining a thin section under a polarising microscope (Fig.6). Agate and chalcedony show a fibrous structure, whereas the quartz in flint, chert and jasper is generally granular. Granular quartz can demonstrate regular banding, but this is not agate (Fig. 7). Nevertheless, it is the colours and rhythmic banding that makes agate the most recognisable of all gemstones. Fig. 2(a) and (b) show agate in limestone from Tepee Canyon, South Dakota, USA. Note that both agates are surrounded … Read More

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Collecting minerals around the world

 Trevor Devon (UK) At some time, I suppose we have all collected rocks or minerals when we were travelling to new places, mostly as mementos, but nothing quite beats the buzz of collecting specific minerals from classic locations with like-minded colleagues. This type of collecting implies you know something of the geology and mineralogy of the location, what sort of rock to explore (often with a sledgehammer to start with) and what colour and shape the minerals are likely to be found in. Of course, it helps to travel with colleagues who have been there before and can show you what to look for. That is one of the reasons why I joined the Sussex Mineralogy and Lapidary Society (SMLS) a few years ago. Fig. 1. Behind the scenes at the mineralogy department of the Royal Ontario Museum, Toronto. Since 1980, SMLS has conducted trips to many parts of the world, including the USA and Canada, India, Namibia in Africa, and several countries in Europe. Such trips usually attract around a dozen or so participants and are often organised with a bit of tourism so that non-mineralogical spouses can join in. I have been fortunate enough to join recent SMLS trips to Cornwall, Isle of Skye, India, the South of France, the USA, Canada, the Caldbeck Fells in Cumbria, and Bulgaria. Perhaps I should start with the basic question of why I collect minerals. First of all, I think some of us are born collectors – for example, I collect … Read More

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