NOTE: This cache is not available from November - April each year and will be disabled.
Unexpectedly, I found myself taking a trip down to Fort Erie. While conducting business unrelated to geoaching, I stopped for a moment and spotted this monument. And then I spotted the potential for an Earthcache....
If you're going to be delayed in sending your answers, please post a note and amend that to a find once your answers have been sent and confirmed - I'll read and respond to all submissions. If in a group, then each person must post their own photo, and it must be clearly indicated in the log & responses sent who the team members are. Thanks.
A brief history:
Built by Niagara Parks in 1939 in the historic town of Fort Erie, Mather Arch is a beautiful monument surrounded by manicured gardens. It is dedicated to the architect of the International Peace Bridge.
At first glance, the arch is noticed for its dedication in remembrance of the World Wars. Upon closer inspection, the unique geological aspects become clearer.
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"Black Granite"
In the construction industry, black rocks that share the hardness and strength of granitic rocks are known as black granite. In geological terms black granite might be gabbro, diabase, basalt, diorite, norite, or anorthosite.
Gabbro is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".
Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer grained equivalents of gabbro are called diabase (also known as dolerite), although the term microgabbro is often used when extra descriptiveness is desired. Gabbro is dense, greenish or dark-coloured and contains pyroxene, plagioclase, and minor amounts of amphibole and olivine. Quartz is found in amounts less than 5%.
In Ontario, gabbro is found in The Lavant Gabbro Complex, which lies northwest of Perth, and is purported to be the largest mafic body in the Grenville Province of Ontario. It covers approximately 250 square kilometres. The Lavant Gabbro Complex falls within the Sharbot Lake Terrane (sometimes called the Sharbot Lake domain) of the Central Metasedimentary Belt of the Grenville Province, Canadian Shield. The Sharbot Lake Terrane is comprised of marbles and metavolcanic rocks that have been intruded by gabbroic and granitic plutons. It is bounded on the east by the Maberly shear zone and on the west by the Robertson Lake shear zone (sometimes called the Roberton Lake mylonite zone).
The Lavant gabbroic complex is a composite intrusion, roughly 50 km long and up to 15 km wide. It consists of (i) a voluminous mafic suite, dominated by medium-grained gabbro to diorite, locally showing igneous layering and crosscutting relationships between several compositionally distinct phases; and (ii) a slightly younger granodiorite–monzogranite suite, which forms several small intrusive bodies and dikes cutting the gabbro. The intermediate to felsic phases occur mainly in the structurally higher part of the complex, and also intrude adjacent supracrustal rocks, including the marbles. Metamorphic grade of the complex varies from upper greenschist facies in the north to lower amphibolite facies in the south. Contact relationships are complex, and inclusions, roof pendants and slivers of all the country rocks are present. The gabbro body does not have an extensive contact aureole (an aureole is the zone surrounding an intrusion, which is a mass of igneous rock that solidified between other rocks located within the Earth). A contact aureole is a region in which country rocks surrounding an igneous intrusion have been recrystallized in response to the heat supplied by the intrusion. The widths of contact aureoles are quite variable and partly depend on the size of the igneous intrusion—the larger the intrusion, the wider the aureole. Geologists determined an age of 1224 ± 2 Ma for the gabbroic and associated monzogranitic rocks of the Lavant complex.
Ocellar (orbicular) varieties of gabbro can be used as ornamental facing stones, paving stones and it is also known by the trade name of 'black granite', which is a popular type of graveyard headstone used in funerary rites. It is also used in kitchens and their countertops, also under the misnomer of 'black granite'.
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Slate
When two tectonic plates meet, enormous pressure on the rock causes the minerals to form flat or nearly flat layers at right angles to the direction of the pressure. It's often easy to split foliated rock along these layers.
Geologists grade metamorphic rocks depending on how much the rocks have been baked or squeezed. In low-grade metamorphic rock, the original minerals have changed very little. Usually, they have grown a bit bigger or been squeezed more tightly together. Low-grade metamorphic rocks, such as slate, usually form near the Earth's surface. High-grade metamorphic rocks have changed a great deal. They have formed deep within the Earth's crust where the temperatures and the pressures are greatest. Minerals in the original rock have been flattened and smeared, even melted. The minerals may have changed so much that geologists cannot determine what the original rock was.
Slate is a fine-grained, foliated (layered), homogeneous metamorphic (transformed from an other) rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash. It is the finest grained foliated metamorphic rock. Shales and mudstones are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone and forms the slate.
Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals.
Foliation in slate may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. This is opposite of foliation in for instance schist or phyllite where the foliation is aligned with the sedimentary plane.
Most slates are gray in colour and range in a continuum of shades from light to dark gray. Slate also occurs in shades of green, red, black, purple, and brown. The colour of slate is often determined by the amount and type of iron and organic material that are present in the rock.
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Limestone & Fossils
There are about 40 limestone and dolomite quarries in south-western Ontario but only a few of them have produced significant mineral specimens. The quarries in south-western Ontario produce limestones, dolostones and shales for a number of purposes such as aggregate, lime and cement production, armour stone, flagstone, building stone and many other uses.
The surficial rocks of south-western Ontario were, originally, shallow ocean-marine sediments, laid down in Paleozoic times, and later transformed to limestones, sandstones and shales. Some of the limestones, in turn, eventually underwent dolomitization (changed from, predominately, calcite to dolomite due to percolating magnesium rich brines)and were transformed to dolomite or dolostone. Sub-economic concentrations of Mississippi Valley-type lead and zinc mineralization occur in many of the dolostones of the region. Probably about 250 million years ago in geological history, warm brines circulated through metal bearing shales in the sedimentary sequences, dissolving metals such as zinc, lead, sulphur, iron and strontium and eventually precipitated sulphide and sulphate mineralization into voids in the dolostones or even replacing parts of the dolostones.
The Gasport Formation is a crinoidal dolomitic limestone: the lower part, thick-bedded with large scale cross-bedding (layering within a stratum and at an angle to the main bedding plane), makes an excellent building stone (“Queenston stone”), quarried at Queenston and in Thorold. Queenston limestone is a durable building stone with low absorption and porosity. The stone is blue-grey when quarried and weathers to a warm silver-grey within ten years. It has been partially dolomitized and well cemented by calcite. It is freestone and can be split or cut into regular blocks of large size. It can even be carved, although it is less favoured as an ornamental stone than other types of limestone.
Fossilization can occur through a variety of methods such as: permineralization (mineral deposits form internal casts of the organism), casts and molds, authigenic mineralization, replacement and recrystalization, adpression (pressed close or lying flat against something), carbonization (conversion of organic material into carbon), and bioimmuration (a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton).
Crinoids inhabited shallow water and grew in dense clusters, sometimes called 'crinoid gardens' because of their resemblance to plants. Long stems were anchored to the sea bed, and held aloft a globose, cup-like structure with radiating arms. The whole animal is formed of many individual plates that usually become scattered when the creature dies. The stem plates are common fossils and the main constituent of crinoidal limestone. Corals: like modern-day coral reefs, the abundant remains of fossil corals in Carboniferous limestone suggest the former existence of warm, clear, shallow and well-lit tropical seas. Corals have a variety of branching and encrusting shapes that provide homes for other creatures and act as a baffle to trap sediment. Different kinds of fossil corals occur at different levels in the limestone, allowing geologists to distinguish between older and younger beds. Brachiopods have become all but extinct in modern seas and oceans, but in the geological past they flourished at the shallow margins of oceans, especially in the Carboniferous. At first they appear little different from familiar modern-day sea shells, but they are in fact quite distinct, with different shell and soft part anatomy. Many brachiopods lived openly on the sea bed, although some lived in seabeds. Two important groups of brachiopods in the Carboniferous are strongly radially ribbed forms, called 'spiriferids' (long hinge-line which is the widest part of the shell), and large, less strongly ribbed forms with relatively plano-convex valves, called 'productids' (round with protruding "spikes"). Fossil annelids (worms): are also common in sedimentary rock. Eunicid worms resemble earthworms; priapulid worms often have a slightly curved body (end); spipanculan worms are quite rare and look like they have fat bodies with a much narrower "tail"; serpulid worm tubes are round and clustered.
Questions - please be sure to complete all responses including the required photo. Sending your responses using the IM feature is preferred.
1. Look at the stone that comprises the arches.
a) Describe the appearance of the stone (e.g. particle size, colour, etc).
b) Using the information in the cache page, identify and measure at least 2 different types of fossils found at GZ. Describe the fossils you see (e.g. shape, relative amount, approximate sizes, etc).
c) Using the information in the cache page, explain in your own words how the fossils you identified in 1b were formed.
2. Find the paving stones used between the arches (WP1). Study them closely and describe the properties of the rock with respect to texture.
a) Is it fine grained or coarse grained? Foliated or not?
b) Is the rock homogenous or can you observe different type of minerals and crystals?
c) What is the colour of the rock?
d) Identify this rock and explain why you think this type of rock was selected for use as a paving stone.
3. Look at the centre of the fountain (WP2).
a) What rock type is it and how do you know? Describe what you see in terms of colour, texture, structure, and any other identifying features.
b) Identify its mineral composition, and estimate the % composition of each.
c) What characteristics does this rock possess that make it suitable for its current use?
4. Finally, to determine that you were actually on site, post a photo of you / your geocaching name / item in front of the fountain with your log. If in a group, then each person needs to post their own unique photo with their log.
Thanks for visiting this EC!
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Sources: http://www.geologyontario.mndm.gov.on.ca/mndmfiles/pub/data/imaging/POP001//pop001.pdf
http://fossilslanark.blogspot.com/2014/10/layering-in-lavant-gabbro-complex.html
https://www.davidkjoyceminerals.com/pagefiles/articles_ontariolimestone.asp
http://www.utsc.utoronto.ca/~eyles/pdf/Niagara%20Field%20Guide%202009%20May%20GAC%20Toronto.pdf