Principles of Sidewalk Geology

Concrete sidewalks are interesting public spaces, or rather public non-spaces. Sidewalks are places of transit, places of transition from here to there, often overlooked because of their functional nature. Yet concrete sidewalks are perhaps the most accessible aspect of urban geology, providing opportunities to study geological material and processes.

Foundation Knowledge

The principles of sidewalk geology begin with knowing the material components that go into making a concrete sidewalk; gravel, sand, concrete, water and air. Gravel can be observed on some surfaces, or in broken edges, and can be identified as local bedrock source.

What gravel is used in your sidewalk?
Where is it from?

Descriptive Studies

Once the material properties of concrete are understood, the field studies can begin. Description and recording of observations are the first steps in the study of sidewalk geology.

Description of sidewalk panels includes identifying textures, colours, and mapping the sidewalk panels. Observations that convey timing of different concrete pours provides a time element, a stratigraphic relationship. How long are the sidewalk panels, are the all the same or estimates?
The concrete sidewalk panel on the left has more pebble aggregate exposed than the one on the right.

What is the texture of the sidewalk panel?
How much gravel is observed on the surface?

Concrete Analysis

Concrete that is used in civil engineering projects like bridges, sidewalks, buildings, is tested and analyzed at engineering laboratories. Concrete is cut and polished to document amount of entrapped air, or put under pressure to see how far water can soak into the concrete.

1) South shore, 2 Central, 3) North shore, and 4) Central Nova Scotia.

By carefully examining the concrete in the sidewalks and structures, the differences in aggregate and sand features can be identified. The type and colour of aggregate (pebble) can often identify where the quarry was located.

Research Question – Does the amount of visible gravel in a concrete sidewalk increase with time? Is the surface of the sidewalk worn down to expose more gravel due to normal erosion?

As part of the analysis of the concrete, the mapping of different ‘types’ of concrete, different pours of concrete that are represented along the walkway.

Chalk can be used to outline a standard sample frame. Perhaps something like a 10 x 10 cm square, that could be made around a wood template.

After drawing a chalk frame around a sample site, a drawing can be made to map the features observed in the sample. Identification of different gravel types may be possible. How can we estimate percent of gravel is visible?

How can we estimate percent of gravel is visible?
A standard scale?

Describing the size, angularity, colour, and rock type of the visible pebbles. Are there some that are more rare, others more common?


As the initial description is completed, a permanent record can be established.

  • Photographic documentation is the fastest record. A scale bar is important, and a specific time of day (direction of light) may improve texture visibility.
  • Although making a mold of the surface may be worth trying as demonstration of principle. Pouring a plaster mold may require a separator to avoid bonding, and wet concrete may produce finer details.

Deeper Questions

As we examine concrete sidewalks more closely, we can also ask questions about what is below the sidewalk.

Draw a sketch of what you know.

  • How thick is the concrete sidewalk?
  • What is below the sidewalk? Gravel?
    How thick of a layer?
  • How far down is the bedrock?
  • How does hydrology (water flow) impact the sidewalk? How far down is the water table?

Sidewalk “fossils”

When concrete is poured into the forms to make sidewalks, the concrete will begin to set after the surface has been levelled and textured with a broom. At that point, there is a period of 4-5 hours that the concrete is particularly receptive to impressions from leaves and footprints of birds, cats or dogs. These hours are a moment in time, and these sidewalk “fossils” are the traces of that moment that remain in the concrete.

There are opportunities to carry out science questions related to the concrete “fossils”. Mapping of traces in urban areas could provide scope of what questions that might be of greatest interest.

By taking photographs and creating maps of urban concrete “fossil” traces, documenting their appearance, location, and identifications and categories, is adopting a science-based framework. The process of a general survey is to document and categorize in order to examine patterns.

What’s the most interesting sidewalk “fossil” you’ve seen?

Note: I am grateful for recent discussions with Carl Mehling and Laura Meyer about concrete sidewalks and the traces left in urban areas. This post was written as a summary of a presentation for the geology class at Auburn Drive High School.







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