Discover the geology
Content provided by Geologist, Chris Fouts
When many of us think about geology and rocks most of us think about finding mineral deposits and extracting resources to produce the modern conveniences we enjoy. Rarely do we realize how much the bedrock geology affects the topography of our landscapes.
Looking out from the scenic spots along the edge of Eagle’s Nest we have an excellent opportunity to think about geology and the landscape. From the Eagle’s Nest lookout, a hill which is part of a system of hills extending to the north, south and east, we observe the York River slowly winding its way through a broad gently sloping valley, bounded to the west by a line of rounded hills. Let’s take a moment to contemplate the view we see. How did it come to be? Why are we standing upon a high point of land here, looking down into a river valley to the west? Why did the river form where it is, why did it not run through the site we stand upon today? Why did the river valley form at all?
Photo courtesy of Sreet-Ings Photography
Erosion is the reason we have the topography we see, but what types of erosion are happening? We live in a comfortable temperate climate, without local deserts, so erosion by wind is minimal. Water is the major player, both as liquid and as ice.
Everywhere that rain lands we will observe water erosion. Rain falls on hillsides, plains and valleys, running downhill and collecting in low points. Collecting as creeks and streams, it gains energy to carry sediment particles. These particles scrape the bedrock and slowly erode a groove. Because the energy is greatest along the riverbed that is where the greatest erosion occurs. Rivers cut down into their riverbed and form what is called “V-shaped” valleys. Steep sided slopes leading down to the river or stream. As we look out into the York River valley what is seen is not a “V” shaped valley but rather a “U” shaped valley. This is formed when water frozen in the form of ice flows through a valley scraping the valley floor and sides evenly with sand and gravel and boulders that are frozen into the bottom of the ice sheet. Similar to a huge sheet of sandpaper scraping over the landscape. Instead of the erosion action being concentrated only on the river bed the erosional action is spread all along the valley floor and lower sides, creating a “U” shaped valley.
The gently sloping “U” shaped valley, along with the well-rounded hills, are a clear indication of glacial action. Ice sheets which covered this region, up to a kilometre or two thick, a mere 100,000 years ago dramatically impacted our landscape.
What is granite and how did it form?
Okay, so river erosion and glacial erosion formed the York River valley, but why there, not slightly east or west? For this answer let’s look at the bedrock. The first step is to have a close look at the ground we stand upon. If we look closely the ground around us we will see that the bedrock is granite. If we prospect this entire hill we will find that it is consistent from the very foot of the hill to the very peak where the former Natural Resources ranger tower once stood. If we take an opportunity to prospect further afield we will find that the bedrock that surrounds the hill is mainly metamorphosed sedimentary rock (such as marbles, amphibolites and schists). Granite is a rock type composed of feldspar and quartz with minor amounts of mica. Feldspar and quartz are relatively hard minerals. They are difficult to scratch, or erode, making granite highly resistant to weathering. Conversely, the metamorphosed sedimentary rocks, (marbles, schists and the like), are relatively soft, and erode quickly. The bedrock around the Eagles Nest is subject to faster erosion than the granite which makes up the hill, thereby making it likely that the river will carve its path around the granite, not through it. As the softer enclosing bedrock continues to erode over millions of years the granite is uncovered, exposed to surface, and over time becomes a high point of land relative to the land around it.
But one other large clue is visible to us as we deal with these questions. The face of the Eagle’s Nest displays wonderfully sharp cliff faces. With eons of wind and water erosion, and a great Ice Age to dramatically shape the landscape how do we still have such a sharp drop-off along the Eagle’s Nest edge? Nature has worked hard to soften the landscape and grind everything down to a flat plain, but this site still exhibits a dramatic cliff faces. The reason lies in in plate tectonics. The movement of plates of the Earth’s crust. The cliffs of the Eagle’s Nest highlight a fault line that runs roughly north-south along the York River valley. The fault line is difficult to determine south and north of the Eagle’s Nest, where the soft bedrock has eroded down on both sides of the fault line to flatten the landscape, but where the fault passes through the Faraday granite body, the land is not eroded down as much, and the “break” is clear to see. The Earth’s crust in the valley has sunk in relation to the crust on the Eagle’s Nest and the crust on the west side of the valley. So not only has softer rock caused to the river form where it has, a fault has deformed the Earth’s crust and formed a low area where water will be drawn to.
The entire hill is composed of granite, the Faraday granite to be specific. The Faraday granite is a granite mass which covers over 50 square kilometers in the north and northeast portions of Faraday Township, the Town of Bancroft, and eastward just past Clarke Lake. This granite mass is termed a batholith. Granite magma seeped upwards through the Earth’s crust and pooled as this large mass.
The Faraday granite is mainly pink microcline (potassium rich) feldspar, Whyte to pink albite (sodium rich) feldspar, and milky Whyte to clear quartz, with minor black annite (iron rich) mica, with minor patches of dark green coarse-grained pyroxene. It is dated between 1250 – 1240 million years old, and formed at a depth of about 10 – 15 kilometers in the Earth’s crust.
Granite is the most common rock type found on the Earth’s surface. It is an igneous rock type composed of a minimum 30% feldspar, 30% quartz, and 5% mica. Other minerals may be present, and often are, however feldspar and quartz are the major components. Being relatively “hard” minerals, they are resistant to erosion and give granite its enduring quality. An igneous rock means it was “born of fire”. It has cooled from a molten state from magma which rose up through the Earth’s crust.
Why is some igneous rock dense and dark coloured, (like basalt), while some is light coloured and less dense in weight, (like granite)? All magma starts as molten material in the mantle. It depends upon how the magma finds its way to the surface. When there is a clean break in the Earth’s crust, (such as where two plates are pulling apart, or where there is a hotspot melting through the Earth’s crust), magma can travel relatively quickly to the Earth’s surface. As such, the magma changes very little and is compositionally very similar to magma deep the mantle, which is dark coloured, iron-rich, metal rich, and dense. We see this in examples such as the Mid-Atlantic ridge spreading zone, and hotspots like Hawaii.
Should the magma travel more slowly up through the Earth’s crust, pooling in spots in the crust for long periods of time, then the nature of the magma changes. While pooled in chambers the magma begins to separate out heavy minerals from light minerals (less dense and more dense). Dark coloured iron-rich minerals, along with metals such as copper, silver, gold, lead, and the like are dense and tend to sink to the bottom of the magma chamber. When the chamber is activated and magma resumes its trip towards the surface, only the top material travels, the bottom material is often left behind. So as magma migrates to the surface slowly it evolves from dark metal rich dense magma to lighter coloured, less dense metal poor material, such as granite.
Keeping the above described processes in mind, what we tend to find is that dark basalt rock is found at divergent plate boundaries and hot spots, while granite is formed at convergent plate boundaries, where two tectonic plates are colliding together.
Most often when two tectonic plates collide one will subduct, or descend, under the other plate. The subducting plate plunges into the Earth’s mantle and begins to melt. Not all minerals melt at the same temperature. Lighter coloured, less dense, iron-poor minerals melt at a lower temperature. As the plate descends, the minerals that compose granite are the first to melt and they separate from the descending plate and rise through the crust. This process is what we believe has formed the granites we see on the Earth’s crust today. Therefore, we can assume that there has been colliding tectonic plates and subduction occurring this region in the past, and as we collect further data we conclude that this was clearly the case.
Just over one billion years ago the southeastern coastline of the proto-North American plate ran roughly through present day North Hastings, stretching northeastwards through Pembroke and southern Quebec, and southwestwards through Haliburton and southwestern Ontario. Subduction was occurring along this coastline, similar to what we see along the west coast of South America today. Subduction of oceanic crust created folded and faulted crust forming a coastal mountain range dotted with volcanoes. Ultimately the collision of continental plates, (that would later form the African and European plates), pushed these mountains up to a point where they would have reached heights as high as the present day Rocky Mountains or even the Himalayas.
The oldest rocks of the immediate area are the marbles, schists and amphibolites which make up most of the bedrock. These rocks formed from sediments deposited in shallow water coastal areas about 1.3 to 1.4 billion years ago. Marbles, schists and amphibolites are metamorphic rocks. Metamorphic means that they have been changed from what they started out as, into something else. High heat and pressure from being buried deep in the Earth’s crust recrystallizes and transforms these rocks. Limestones become marbles, shales and siltstones become amphibolites and schists, sandstones become quartzites. After these sediments are lithified into stone and further metamorphosed, they were intruded by different magmas. In some cases, dark coloured gabbros, (between 1290 – 1250 Ma), at other times light coloured granites and syenites. (~1250 – 1240 Ma)
Granite: Light coloured plutonic igneous rock composed of a minimum 30% quartz, 30% feldspar, and 5% mica. The most common rock found at the Earth’s surface.
Basalt: Dark coloured igneous volcanic rock composed of feldspar, with minor pyroxene, amphibole and occasional mica. A relatively iron and metal rich rock type. The most common rock type in the Earth’s crust.
Marble: Composed mainly of recrystallized calcium carbonate (calcite), formed by metamorphizing limestone.
Amphibolite: A dark coloured metamorphic rock composed mainly of amphibole, commonly containing minor amounts of dark mica.
Schist: A metamorphic rock mainly composed of subparallel flakes of mica, often with varying amounts of quartz, feldspar, and amphibole. Often distinguished by flakey texture.
Syenite: Igneous rock composed of feldspar, minor amounts of mica, with trace to no quartz present. In simple terms it looks like a granite without quartz.
Gabbro: Dark coloured plutonic igneous rock composed of feldspar and pyroxene, with or without amphibole, and minor amounts of mica.
Volcanic: A fine grained igneous rock which has cooled quickly at the surface of the Earth.
Plutonic: A medium to coarse grained igneous rock which has cooled slowly within the Earth’s crust.
Igneous: Any rock which has been “born of fire”, that is, cooled from magma from below the Earth’s crust.
Sedimentary: A rock formed from sediments laid down in water, or as sand dunes, and compressed or cemented together.
Metamorphic: Any sedimentary or igneous rock which have been altered or recrystallized by extreme heat and/or pressure, often due to burial at deep depths.
Intrusive: An igneous rock which forms from magma intruding into pre-existing rock.
Feldspar: Calcium, sodium, potassium aluminum silicate. A family of minerals which make up the most common mineral type on the Earth’s surface. Varieties include albite, orthoclase, microcline, oligoclase, labradorite, among others. Major mineral type in the rocks granite, syenite, gabbro, and basalt.
Quartz: silicon dioxide. Hard trigonal structured mineral which can occur in many different colours due to trace amounts of other elements entering the crystal structure. Varieties include milky, smoky, rose, amethyst, citrine, chalcedony, among others. Second most common mineral on the Earth’s surface
Mica: potassium calcium, sodium, magnesium, iron aluminosilicate hydroxide. Third most common mineral found on the Earth’s surface. Refers to a family of minerals with a common hexagonal structure and a habit of forming thin flexible plates. Whyte coloured potassium rich muscovite; brown coloured magnesium rich phlogopite, and black coloured iron rich “biotite”, recently renamed annite.
Amphibole: Refers to a family of common iron and magnesium rich rock forming minerals which show distinctive cleavages meeting at 60 and 120 degrees to each other. Generally dark coloured – black to dark green to green, grey and Whyte. Includes minerals such as hornblende, kataphorite, actinolite, and tremolite.
Pyroxene: Referring to a family of common iron and magnesium rich rock forming minerals which show two good cleavages planes meeting at 90 degrees to each other. Dark coloured - black to dark green to green. Includes minerals such as augite and diopside, among others.
Subduction: At a convergent plate boundary where one plate descends under another plate. The subducting plate descends into the Earth’s mantle and melts.
Tectonic plates: Large pieces of the Earth’s crust which are constantly being created at spreading centres (mid-ocean ridges), and destroyed at subduction zones.
Batholith: A large mass of granite formed by magma pooling in a magma chamber, on the scale of 10 to 100 square kilometers.
Hastings St. N
Bancroft | ON | Canada