Bricks and Mortar – The Rocks That Make Up The Rockies

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The Rocks that Make Up the Rockies

Many people, upon first seeing the Rockies assume they are made of very hard rocks like granite. They believe (mistakenly) that the mountains are either volcanic in origin, or that the rocks (like granite) had a molten genesis. With the exception of a few isolated pockets of igneous (formerly molten) rocks, the Canadian Rockies are composed exclusively of layered sedimentary rocks. These include limestone, dolomite, sandstone and shale, amongst others. Worldwide, sedimentary rocks cover approximately 75% of the worlds surface. Of this, approximately 50% are shales, 30% sandstone and 20% limestone.

Sedimentary rocks have a unique method of deposition – one layer on top of another. This seemingly simple arrangement can be extrapolated to assume that the rocks nearest the surface will always be younger than rocks deeper down. Digging through the layers, geologists can analyze their composition, and determine much about the climate and landscape during the time of their formation.

In the mountains, this organized arrangement has been shattered. Older rocks have been piled up on top of their younger neighbours. They have been bent, folded, cracked, and eroded. The original order is often impossible to determine, however geologists have done an amazing job of reconstructing the various layers. By knowing the formations, they can estimate the age of the rocks, anticipate how they will react to erosion, and get a better understanding of why the landscape looks the way it does.

Sedimentary rocks result from compression of the layered sediments on the bottom of a large body of water. Differences in parent material, along with the effects of weathering, erosion, transportation and deposition can have a large impact on the resulting layers. They can be divided into two major groupings: inorganic and organic.

Inorganic rocks are those formed by the deposition of inorganic matter. This includes minerals as well as the remains of other older rocks that were eroded away, only to have their individual grains deposit as layered sediments.

Organic rocks are further broken down into chemical and organic origins. This group combines rocks formed from the remains of living organisms along with rocks resulting from several chemical processes. These include the limestones and dolomites that form many of our mountain summits, along with other valuable resources like coal.


Many small towns in Alberta began as little more than a mine site around which a small community developed. But beyond its immense historical importance, the story of coal’s formation itself is fascinating.

Coal began in large marshy bogs. At the bottom of these bogs, plant material accumulated, and if conditions were just right, gradually transformed into peat. Peat can be defined as an accumulation of partially decomposed plant material containing around 60% carbon and 30% oxygen. In many area’s, peat is used as a low grade fuel source. If the peat is subsequently covered with other inorganic deposits (sand and mud), the pressure from these overlying sediments squeezes out the water and organic gases. This increases the percentage of carbon, and begins the transformation from peat to coal. It's a slow process, but eventually the mixture becomes almost pure carbon – coal.

If the process repeats itself several times, there may be several successive layers of coal. If you take a thin section of coal, and look at it under a microscope, you can see a mass of plant debris: pieces of bark, wood, leaves, cells, spores and algae all floating in a black jelly.

In the Bow Valley, coal was first reported by George Dawson in 1885. The seams were substantial, and he named the formation the Cascade Coal Basin. This deposit stretched from north of Banff to south of Canmore. It was the Canadian Anthracite Company of Eau Claire, Wisconsin that opened up the mines in Canmore and Anthracite (west of Banff). They beat the CPR to the punch and were able to supply them with coal until they opened their own mine at Bankhead.

Of the three Bow Valley mines, Canmore's was the most viable. Its coal was much easier to remove as it was not as steeply bedded. The coal at Anthracite and Bankhead (Mine #80) had much thinner beds as well. Canmore's mine proved itself by operating until 1979, while Bankhead closed in 1922, and Anthracite lasted only a few years, closing in 1904.

In the Rockies, the area’s natural history has always influenced the human history. Originally, rich fur and mineral resources brought people to the area. Today, it’s the landscape and scenery that attract visitors from all over the world. If you'd like a taste of the past, take a walk on the Georgetown trail at the Canmore Nordic Centre, or drive up to the old site of Bankhead and either walk the old townsite or climb the C-Level Cirque trail. You can still feel the history and see some of the old mine remnants.


In the Canadian Rockies, limestone forms most of the resistant ridges and summits. Often interspersed with layers of shale and sandstone, limestone is more resistant to erosion. This leaves it forming the upper-most layer on most of our peaks.

Limestone is composed of calcium carbonate (CaCo3), and can be the result of both organic and non-organic processes. In marine environments, many plants and microscopic animals are able to extract calcium carbonate from the water to aid in the formation of hard outer shells. As they die, their shells litter the ocean floor, layer upon layer, until they result in beds of limestone. This also explains why limestone beds are often very rich in fossils.

Limestone can form entirely without the help of organic material. In warm climates, calcium carbonate can become concentrated in seawater. As it reaches a critical point, it begins to precipitate out in tiny grains the size of sand. These grains are known as oolites, and as they are moved by the ocean currents, additional CaCo3may deposit upon them. As they settle on the bottom, they are sorted according to size. They may, in many deposits be mixed with organic limestone.

In very quiet water, the CaCo3 precipitates as linear crystals. These settle to the bottom and accumulate in the silt. These crystals pack tightly together to form a dense limestone known as micro-crystalline limestone. The density is such that high magnification is required to make out individual crystals.


If you take limestone, and add some magnesium, you end up with dolomite. Chemically it is called calcium-magnesium carbonate or CaMg(Co3)2 , and is generally caused by the substitution of some calcium for magnesium in limestone. In rare instances, it can precipitate directly from seawater. In general it’s harder than limestone and may replace it as some of our summit layers.


Sandstone is an easily identified rock made up of individual grains of sand cemented together by chemicals such as calcite, silica, or iron oxide. It is often very hard, forming much of the early skyline of Calgary earning it the nickname “Sandstone City”. Today much of the sandstone has been replaced by concrete and glass, but many of the older buildings still pay homage to this durable building material. In the foothills, the ridgetops are generally made up of sandstones underlain by layers of shale.

Sandstone can contain many minerals, but the most common, and the most durable, is silica. Other minerals include feldspar, mica, and olivine, however excessive transport will usually remove these unstable minerals and concentrate the silica. Every cycle of erosion and deposition tends to purify the sandstone, slowly removing other minerals from the mix. Pure silica sandstone has generally gone through several cycles of erosion and deposition.


Shale is familiar to most of us. Often if forms thin beds of very fine grained, easily broken rock. It is the most common layered rock we know and is found around the globe. It’s grains are generally finer than 0.166 mm, making them impossible to distinguish with the naked eye. It is usually found beneath a protective layer of limestone in the Rockies. If the limestone summit eroded, the underlying shale beds would quickly follow. Very crumbly and brittle, they cannot withstand much weathering.

Shales contain many differing characteristics, and these reflect their origin. In shallow, still lagoons, dark shales, high in organic material form. On the other hand, tidal flats, stream channels, and flood plains often result in iron rich shales. These oxidize when exposed to oxygen and result in the red shales common in many area’s, including Red Rock Canyon in Waterton Lakes National Park.

Past Environments In Which Our Sedimentary Rocks Were Formed

When one looks up at the towering sentinels that make up the Rockies today, it’s difficult to imagine this area looking any different. Reality can be stranger than fiction though, and by realizing the aquatic nature of these lofty peaks, we begin to imagine a clearer picture of these rocks formative years. Although the timelines may seem incomprehensible, the critical point is that the same processes occurring a billion years ago are still taking place today.

The early days – 570 to 410 Million Years Ago

During this period, known as the middle Cambrian, the western edge of present day Alberta also marked the western margin of the continent. In the shallow waters of the Pacific Ocean, the greatest explosion of life ever seen on the planet occurred. The Cambrian Explosion, as it is known, saw the evolution of all major groupings of animals on the planet today – including arthropods, molluscs, worms, starfish, jellyfish, and even chordates (back-boned animals). It was also much warmer then. Alberta almost straddled the equator. In time, the Pacific drowned much of southern Alberta. By the end of the Ordovician (440 million years ago), most of the continent had been flooded. Beneath the shallow waves, and along the margin of the continent, extensive reefs of coral formed. At the same time, the hard shells of tiny single-celled creatures settled to the bottom to form limey muds. Later, the reefs, now fossilized as limestone, were thrust upwards through mountain building. Look to the steep slopes of Castle Mountain for an excellent example.

This period also saw the first predators. With the advent of hunters, the hunted had to get creative. They developed shells of calcium carbonate, chiton (similar to fingernails), and silica. These hard parts later formed the raw materials for fossils. On creative solution to predation was to leave the water altogether. During the Silurian (440-410 million years ago), the first vascular plants arrived on the shores of the planet’s oceans. The first fish began to swim the oceans around 470 million years ago.

Alberta’s Billion-dollar Rocks – 410-250 Million Years Ago

As Alberta continued to be inundated, reefs continued to form. As time went on, new forms of life began to swim in the oceans and invade the shorelines. During the Devonian, the period between 410 and 360 million years ago, many of our limestone summits were formed as coral reefs. One of the most common reef building creatures, stromatoporoids, created the porous limestone we see today. These reefs later formed the natural reservoirs for 60% of Alberta’s oil and gas reserves. This has earned the stromatoporoid limestones the nickname Alberta’s billion-dollar rock! It is these same Devonian limestones that Canada LaFarge, located on Lac Des Arcs, quarries to make cement.

The limestone summits of our most famous mountains, Three Sisters, Rundle, and Cascade, are made of more recent deposits, in the area of 300 million years old. During this entire period, the continents constantly moved in relation to one another, until they joined one large land-mass known as Pangea. While reefs continued to form off-shore, plants thrived on land. This was also the age of reptiles as vertebrates severed their ties with the oceans. Giant dragonflies hovered. Unfortunately for this diversity, the Permian ended with the largest global extinction in the history of the planet. Within a short period, 95% of the worlds species were wiped out (compared with only 65% during the Cretaceous extinction).

The Age of Dinosaurs – 250 to 65 million years ago

After the Permian die-off, a few of the surviving reptiles re-entered the oceans. Soon a diversity of aquatic reptiles began to hunt the depths. Many of the best fossils of this period are found near Wapiti Lake, northwest of Jasper National Park. During most of the Triassic, the western part of the province of Alberta was submerged, while large rivers drained the remainder of the province. By the end of the Triassic, Pangea began to break apart, and the continents once again began to drift. Around this same time, a series of asteroid impacts, at least three worldwide, resulted in another massive global die-off.  The stage was now set for the arrival of the “terrible lizards” – the dinosaurs.

The Jurassic saw the earliest true dinosaurs, the long necked Brachiosaurus and the carnivorous Allisaurus. The plated Stegosaurus provided a formidable opponent, while duck-billed dinosaurs preferred the swamps. Warm waters off the coast turned the climate of Alberta warm and humid. Approximately 175 million years ago, the first tectonic collisions off the west coast began to add British Columbia to the edge of the continent. At the same time, pressures from the collision began to crumple and fold up the Western and Main Ranges. On the coast, volcanoes erupted. Further inland, formerly submerged layers of rock were pushed on top of younger deposits, thus beginning the piling up of the Rockies. With this great uplift, the oceans were finally squeezed out of Alberta.

To the east of the mountains, extensive marshes existed. Coal was formed as organic materials were buried within the swamps. Later, these deposits would provide much of the provinces power supply.

Conditions 65 Million Years Ago 

By the time the front ranges rose around 70 million years ago,  much of Alberta had become a wet, humid dinosaur paradise. The ocean began to invade from the north, while huge amounts of sediment flowed eastward from the newly minted mountains. By the middle of the Cretaceous, flowering plants had also arrived on the scene. Although this period resulted in extensive beds of sandstones, siltstones and shales, most were later removed by erosion. The Paskapoo Sandstones are of this period.

Rocks Can Talk – Looking at the Rocks

Rocks have the ability to tell us an epic adventure about past environments. We’ve discussed already how the area now occupied by the Canadian Rockies was once a large inland sea. Today, eons after the waves have receded, we can look at the rocks and see evidence of changing climates, former beaches, dry washes, and even former aquatic residents.

The best way to study the mountain geology, is to seek out and search for some of the signs of previous environments. These are often things we can recognize around us in our everyday life – signs of wind or water deposition, mud cracks, or even ripples in the sand near our favourite beaches. In many cases, these common reflections of the present have been preserved permanently in the rock structure.

Graded Bedding

When muddy water flows into a lake or ocean from a river, it may flow through the clear water without mixing with the it. This same principle, called a turbidity current, occurs as muddy water moves down steep shelves in the ocean, bringing sediments rapidly to the deeper water. The lack of mixing is caused by the density of the turbid water.

As a turbidity current moves, it slows down gradually. As it slows down, the heaviest materials are deposited first, followed by those progressively lighter. Finally, the very lightest sediments are deposited. Visually, you’ll see a layered appearance of sediments sorted by size. There may be several sorted layers atop each other. This was caused by successive turbidity currents having deposited one after another.

Cross Bedding/Current Bedding

If currents of wind or water flow across loose sediments, the particles will often take on the form of ripples or dunes technically referred to as sand waves. These may be the small ripples visible near many beaches, or they may be sand dunes hundreds of feet high caused by wind rather than water. The direction of the current has a great influence on the appearance of the dunes. Normally the windblown side will be smoothly sloping with a steeper slope on the sheltered side. This predictable appearance allows us to identify the direction of prehistoric currents, and get a better idea of past environments.

Ripple Marks - click to see a larger viewIn many shale deposits, ripple marks are clearly visible. Looking like a wavelike pattern on the surface of a piece of shale, it is one of the most common indicators of the aquatic origin of the Rockies.

Mud Cracks

We are all familiar with the mud cracks that form as mud dries after a storm. Mud flats are particularly common on area’s where normally aquatic environments are periodically exposed to air. If these are subsequently buried by later sediments, they may remain preserved in the rock record, only to later surface as fossilized deposits.

Mineral Type

Since sedimentary rocks are the result of a sequential process of erosion and deposition, we can often find a mixture of many minerals trapped in their stony facade. Learning to recognize some of these common minerals can make it enjoyable to study seemingly homogenous deposits of sandstones. The most common mineral is silica, which tends to become more common with each cycle of erosion and deposition. Deposits that have not been recycled numerous times may also contain significant amounts of minerals like feldspar, mica, and olivine.

Grain Shape

The shape of individual grains can tell us the mechanism by which the rock was originally eroded, and perhaps how it was transported. Angular grains are usually the result of mechanical weathering like frost wedging. It also indicates that they did not travel far from where their original parent material. On the other hand, if the grains are round and smooth, they must have traveled quite some distance. They may have been carried in the current of a river, or within a glacier.

Fossil Fuels

We've all heard the term "fossil fuels" and most know that these include oil, gas and coal, but just where did the term come from? All three originate from fossil plants and animals, and hence the name fossil fuels. The main categories, coal, natural gas, and liquid fuels like oil and gasoline, vary in the amounts of hydrogen and carbon they contain. The higher the percentage of hydrogen, the more combustible the fuel. Coal, almost pure carbon, contains an average of 6% Hydrogen, making it a low grade fuel. Natural Gas, and gasoline consist of between 10 and 50% hydrogen.

The creation of petroleum and natural gas differs greatly from that of coal. These fuel sources are almost exclusively found in marine deposits. Scientists believe the remains of microscopic organisms accumulated in seafloor muck and may not have completely decomposed. These remains were later buried to great depths. The heat and pressure associated with these depths slowly transformed the organic material to hydrocarbons. The oil and gas would have flowed upward through the rock layers until trapped by an impermeable layer. The resulting reservoirs of oil and gas remain until erosion of the impermeable layer, or an oilwell, release it.

The Rockies contain many natural storage basins for natural gas. In particular, the foothills contain many natural domes, or anticlines. The domes, if capped with a hard layer of limestone, prevent the gas from escaping to the surface. Moose Mountain, in eastern Kananaskis, now hosts numerous gas wells. The flat top of Plateau Mountain has also acted as a natural storage reservoir.

Concrete and Glass – How Our Rocks Became Skyscrapers

On the picturesque shores of Lac Des Arcs, a massive industrial complex often begs the question “what do they make there?”. Although surrounded by a landscape of majesty and magic, this plant has closer connections to the urban landscape of Calgary. Behind the plant, a large limestone quarry is slowly tearing down the once towering face of Limestone Mountain. This material is crushed, sorted, baked, and turned into cement. The cement is in turn mixed with other materials to create the concrete that towers above the plains in Calgary.

Canada Lafarge, the company operating this massive facility, forms the largest cement manufacturing conglomerate in Canada. After quarrying limestone near the plant, the rock is crushed to make smaller chunks of rock no larger than 5 cm. This is mixed with shale from another nearby quarry to make a precise mixture of calcium, silica, aluminum and iron. Later, this compound will be baked in enormous kilns at 1450°C which form compounds called “clinker”. This is ground up into even finer materials to form cement.

The process continues with additives like fly ash, silica and gypsum flume. It is later mixed with other aggregates and water to form the hard concrete that spurs the construction boom throughout Alberta and the surrounding Rockies.

Hire an expert guide show you these processes in action

All Material © Ward Cameron 2005