Department of Earth and Planetary Sciences
McGill University
Understanding Planet Earth (186-201C)
http://www.eps.mcgill.ca/~venetia/201c
Geologic Field Trip 1
Urban Geology
This program was originally developed by Venetia Bodycomb and David Dolejs. I thank Venetia for
her geological wisdom and for the preparation of this document.
The Building and Ornamental Stones of Montreal
It is amazing how everyday we walk past or over a wide variety of rocks without ever noticing!
Hopefully, this field trip will make us think twice about the materials used to build and decorate the city.
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The use of rocks in a metropolitan area is generally a two stage process:
An earlier stage is defined by the use of building stones quarried from the immediate vicinity or nearby.
The earlier constructions can therefore show us a lot of the rock types that underlie the city because early
settlers could not afford (both in terms of money and energy) to bring building stones from very far
away. For example, Montreal is dominantly underlain by limestone (calcite rock) and so it is no surprise
that this dull grey to brownish grey rock was used for many of Montreal's older buildings, churches and
stone fences before concrete became the norm.
A second and more modern stage is defined by the use of rocks for ornamental facing on buildings
otherwise constructed with concrete and metal. As a city becomes wealthier and the cost of long distance
transport becomes less prohibitive, architects seek out good looking rocks to decorate their buildings.
These facings, which come from localities all around the world, differ from building stones in that they
are slabs only a few centimetres thick, rather than large three dimensional blocks.
What types of rocks are best to use?
Building stones must be made from rocks that can help support a structure and should therefore not have
inherent pronounced weaknesses like large pores, fractures, faults, or metamorphic fabrics (think St.
Francis Dam – to be seen later during the lecture on metamorphism). For this reason, they are typically
homogenous rocks, and if bedding is present, it is oriented parallel to the ground (perpendicular to the
compressive forces) for maximum strength. Thick layers of limestone or well-cemented sandstone are
good choices, as are many types of igneous rocks. However, since most of Earth’s land surface is
covered by sedimentary rocks, and because they are “softer” to quarry than igneous rocks, sedimentary
rocks commonly dominate as building stones.
Ornamental stones do not have to be homogenous since they do not have to support significant weight:
they are sawed into large slices that are cemented or otherwise fixed onto a concrete structure. They are
chosen mainly for their esthetic qualities and their cost. The most common are dark, evenly toned
igneous rocks (like gabbro), lighter granite and syenite, and veined marble (metamorphosed
limestone). The most expensive have fascinating patterns (e.g., marble, coarse-grained granites,
metamorphosed rocks) and colours (e.g., pink feldspar granites, blue irridescent anorthosite).
Side Notes
Anorthosite
One type of anorthosite (a rock composed entirely of feldspar) comes from Nain, Labrador and is quite
famous for its beautiful blue irridescence. The variety of Ca-Na-feldspar in this rock (“labradorite”) has
closely spaced twin planes which selectively reflect light of certain wavelengths. The result is impressive
blue irridescence in the sunlight that appear to a passing observer as "flashes".
Marble
If marble is made of calcite, and calcite is softer than steel, why do we use marble for counter tops? The
answer (I think!) is that the marble is impregnated or coated with something that increases its strength when
used in this manner, but I don’t know much more about it than that!
The Geology of Mount Royal
Mount Royal is one of the Monteregian Hills and the prominent topographic feature of the
Montreal area with a maximum elevation of 254 m above sea level (just a little higher than the restaurant at
the top of Place Ville Marie). The surrounding area is called the St. Lawrence Lowlands and it is
underalain by sedimentary rocks deposited between ~600 and 400 million years ago near the shore of the
ancient Iapetus ocean. The mountain was formed ~125 million years ago as basaltic magma intruded into
the sedimentary rocks and cooled under the surface to form gabbro. As it cooled, the magma "baked" the
surrounding rocks. Remember that the time of the magmatic intrusion, an extra 1.5 to 2 km of rock still
covered the region. These rocks have since been removed by erosion and so we are only seeing “part of the
story”.
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Calcite (CaCO3) is soft (scratched by a knife) and easily dissolves (fizzes) in weak hydrochloric acid
(HCl; H=hydrogen, Cl=chlorine). The reaction between the acid and the calcite produces water (H2O),
carbon dioxide (CO2) and dissolved calcium ions (Ca2+).
Magnetite (Fe3O4) is a black metallic mineral and will attract a magnet.
Anorthosite is like a gabbro, but contains mainly Ca-rich feldspar with very little pyroxene (see above).
Stop 1: PSE Library (McGill University)
The bulk of the building was constructed using blocks of a grey rock.
a) First determine if this rock igneous or sedimentary, then name it. What is the mineral that makes
up this rock? Write a reaction for the test we used to determine this mineral (you can write this
reaction later).
b) What fossils can you find in the rock? (there are three types: chose from attached fossil page).
What age bracket can you give this rock?
c) Describe how both the rock and mineral formed. Is it a clastic, chemical or biochemical rock?
What problems did you encounter trying to place it into only one of those three categories?
d) How is bedding defined? (e.g., layering is poorly developed but visible nonetheless, so what is
the primary difference between layers that make it possible to distinguish one from the other?)
e) There are some darker, roughly horizontal wiggly lines called stylolites. What are stylolites and
how do they form?
f) Looking at the intricate carvings at the base of the columns, you will notice how much more
worn down they appear compared to those above the door and beaneath the overhang. Why is
this? Write a reaction to illustrate your answer (hint: it’s one of the weathering reactions from
the book; do this reaction later).
g) This is the same type of rock that underlies the Montreal area and may have been excavated from
the Miron Quarry (now a garbage dump). What can we infer about the ancient depositional
environment and climate at the time of sediment deposition? Would you expect to find this
sediment (and therefore this sedimentary rock type) to be forming in the St. Lawrence river?
Why or why not?
Stop 2: Optimum Health Food Store (corner Sherbrooke and Aylmer?)
Also observed outside the Banque Laurentienne du Canada (1981 McGill College).
Look at the slices of rock used to decorate the outside of this building. It is made of a coarsegrained dark grey rock with irridescent blue patches.
a) Is this a sedimentary or igneous rock? Name it.
b) This rock is nearly “mono-mineralic”, meaning that it is essentially composed of one mineral (in
this case, the mineral displays blue irridescence). Name the mineral. What criteria did you use
for identification?
c) What other minerals are present? What criteria did you use for identification?
Stop 3: La Tour Industrielle Vie (2000 McGill College)
This building is covered with a pinkish rock.
a) Is this a sedimentary or igneous rock? Name it.
b) Identify and list its minerals.
c) Where and how would a rock like this form?
Stop 4: Maison Sedgewick (2200 McGill College)
Compare this pinkish rock to the one at Tour Industrielle Vie.
a) What is the principle textural difference at this stop? What caused this difference? Make a
sketch.
b) Did this change in texture take place in the upper part of the crust? Why, or why not?
c) There are some diffuse tabular zones containing coarser but more homogenous material. These
zones cross-cut the linear fabric of the surrounding rock. Use the principle of cross-cutting
relationships to determine if these tabular zones are older or younger. If I were to tell you that
the texture in these zones is homogenous because they crystallized from a partial melt of the
surrounding rock, what do you think caused the rock to melt? (hint: recall that there are three
mechanisms to melt rock -- heating it up, depressurizing hot rock, or adding fluids)
Stop 5: Place Montreal Trust
This entire building is faced with a beautiful pink igneous rock.
a) Name the rock.
b) Identify its minerals.
c) Could this rock have been part of a batholith or not? Why or why not?
d) What type of rocks might have been partially melted to produce the parent magma of this rock?
Hint: we have examined partial melting of two rock types -- mantle and sedimentary (continental
crust).
Stop 6: Place Ville Marie Walls
All the walls and pillars inside Place Ville Marie are faced with a light greyish brown to medium
brown rock with abundant millimetric to centimetric white veins, and very thin (< 2 mm) black
“seams”. The brownish rock represents the original rock type, and the coarse white material in the
fractures was formed at a later time (principle of cross-cutting relationships). This is an example of
“hydrothermal metamorphism”. That is, the rock has undergone textural changes during infiltration
by hot fluids [hydro=water, thermal=heat, therefore hydrothermal=hot water!]. Notice however that
the metamorphic recrystallization has not resulted in a significant increase in grain size.
a) Name the rock. What is the main mineral that makes up this rock?
b) What mineral fills the fractures to form white veins? Where did it come from?
c) What is the black material in the seams? How did the seams form?
d) Make a sketch of the rock showing both the white veins and the and black seams.
e) Do you think this rock formed in the upper or lower part of the crust? Give two reasons.
A continuation of this stop: A.L. Van Houtte restaurant on McGill College. The tabletops are made
of fully recrystallized white marble. Notice the blackish streaks – these are similar to those in the
walls at Place Ville Marie.
Stop 7: Place Ville Marie Floors
Notice that the floor is made from three different rocks: light grey-white, medium grey, and dark
grey. Since these rocks are used for the floor, we can assume that they are fairly common and
inexpensive ornamental stones. They can be found lining the floors of most metro stations as well.
a) Name the magma type that formed each of three rocks (basaltic, andesitic, rhyolitic) and give
the appropriate rock names (hint: check the grain sizes).
Stop 8: The Bay (corner Union and Ste. Catherine)
A strong red sedimentary rock forms the building stones of the front part of the building closest Ste.
Catherine. The rest of the building is constructed with an artificial red stone that attempts to match
the natural rock.
a) Name the red rock (look at our hand sample). Is it clastic, chemical or biochemical? What is the
principle mineral?
b) Although we cannot touch or test the rock directly, you can still deduce the most likely kind of
cement that holds this rock together. What do you think it is? (Your answer should also explain
the colour of the rock. Remind me to tell the Edinborough anecdote about removing the soot
from the buildings.)
c) Look at the blocks on either side of the first window on the left. What are the sedimentary
structures visible? Using your knowledge of the mineralogy of this rock and the sedimentary
structures it contains, briefly describe the degree of weathering and transport (maturity) and the
environment of deposition for the sediment that later became compacted and cemented to form
this rock.
d) Look at the church behind you. What is the roof made with, and what is the weathering process
that has changed its colour. What does that type of weathering mean at the atomic level of the
metal? (Weathering will be addressed during Thursday’s lecture)
Stop 9: 2020 University (inside; corner Presidnet Kennedy and University)
The walls and floors of the 2020 entrance are lined with a sedimentary rock called “travertine”
which forms in caverns and around hot springs. This rock also lines some metro stations (e.g.,
McGill metro) as well as some parts of the MacDonald Engineering building.
a) What is the rock-forming mineral in travertine?
b) Is it a clastic, chemical or biochemical sedimentary rock? Why?