Display rocks 7, 8 and 9 may look like lumpy concrete but they are actually fossilized structures of stromatolites in middle Ordovician age limestone.

Photo:  Metcalfe Geoheritage Park Committee.

Samples courtesy of Duncan Abbott.  Transportation to Metcalfe Geoheritage Park courtesy of Oliver Toop.

Photo by Karen Thompson.

Display rocks 7 + 8 + 9 were collected at the same Almonte location during backyard excavations.  Each has unique features.  Look for nested circular patterns in the rocks.  Paleontologists refer to these ancient forms as biosedimentary structures rather than fossils.1

What are stromatolites?

Stromatolites, from the Greek words ‘stroma’ (mattress, bed) and ‘lithos’ (rock), are thin biofilm layers of cyanobacteria that build up during sedimentation, forming boulder-sized columnar, branching, domal or conical structures. Did you know that, even though fossilized stromatolites are one of the oldest forms of life ever found on Earth (from over 3 billion years ago), we can find living ones even to this day? Also, paleontologists think that cyanobacteria were likely responsible for transforming the Earth’s initially oxygen-poor atmosphere to its present oxygen-rich state1”.


Unmodified public domain photo by Alicejmichel via Wikimedia Commons. 

Living stromatolites from in and around the Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia.


Unmodified public domain photo image By Happy Little Nomad via Wikimedia Commons.

Living stromatolites from in and around the Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia.

Now that you have seen the rounded forms of modern stromatolites at Shark Bay, take a second look at the rocks on display here at Metcalfe Geoheritage Park. These specimens are actually upside-down so, instead of the upper outer dome, you can see the internal fossilized structures.

Luckily for local area residents, we can also find numerous in-situ right-side-up fossilized stromatolites in the limestone rocks beneath and upstream of the Champlain Bridge in Gatineau, Quebec.

When and where did the stromatolites in the display rocks live?

The stromatolites in specimens 7, 8 and 9 lived in a warm and shallow inland sea, approximately 460 to 470 million years ago, during the middle Ordovician Period of the Paleozoic Era. During that time, ocean levels were very high, many types of marine life were evolving, and Ontario was not even located where it is today. The core of the North American craton, Ontario along with it, was actually located on the equator as part of a continent named Laurentia.


Unmodified public domain image by Fritz Geller-Grimm via Wikimedia Commons of an Ordovician seafloor diorama at the National Museum of Natural Hsitory in Washington, D.C., U.S.A.

The shallow, tropical seas that covered the Almonte region during the Ordovician Period were rich environments for a variety of early marine animals.

In what current geological setting is the limestone from the display rocks found?

Limestone rock is made up principally of calcium carbonate, CaCO3, that was initially deposited as lime muds, oozes, and shells in ancient oceans. The limestone of specimens 7 – 8 – 9  is part of the Ottawa Group of rock strata (layers) which, from youngest to oldest, includes five geological formations:  Lindsay, Verulum, Bobcaygeon, Gull River, and Shadow Lake2.  Our middle Ordovician age limestone is found in a unique region that geologists call the Ottawa Embayment, which is part of the St. Lawrence Lowlands3. The Ottawa Embayment locally coincides with the physiographic region known as the Smith Falls Limestone Plain. Geologists consider the St. Lawrence Lowlands to be a sedimentary platform of generally flat lying and undeformed rocks, which can be hundreds of metres thick, extending down to the Precambrian basement.

Our local limestone plain may be generally flat, but did you know that we also find ourselves within a seismically active area call the Ottawa-Bonnechere Graben? A graben is an area of down-dropped rock flanked by geological faults.


Unmodified public domain image by Kmusser via Wikimedia Commons.

Between the Mattawa Fault to the east and the Petawawa Fault to the west, the Ottawa-Bonnechere Graben extends from Lake Nippising past Ottawa towards Montreal.  The Ottawa region has many geological faults, the most widely known being the Hazeldean Fault and the Gloucester Fault.

If you have Google Earth installed on your desktop computer, you can explore the complex pattern of rocks and faults that form the geological mosaic of eastern Ontario. To do this, download Paleozoic Geology from OGSEarth. Save and open the small .klm file, move it into your Google Earth ‘My Places’ and go exploring. To read more about the geological history of the Ottawa Valley, check-out this Geoheritage of the Ottawa River Drainage Basin chapter. Happy reading.

What is this rock used for?

The carbonate rocks of the St. Lawrence lowlands are a source of aggregate, cement, and building stone3. They are also associated with petroleum, salt, and gypsum deposits. Interestingly, the Ottawa Group limestone has been used as building stone for some of our most historic landmarks.


Unmodified public domain photo by D. Gordon E. Robertson via Wikimedia Commons.

Rock from the Lindsay Formation was used to build the Bytown Museum (above) and the Lockmaster’s Station.  Bobcaygeon limestone forms the Arts Court and Notre-Dame Basillica in Ottawa.

Not only are small pockets of the local limestone used for aggregate and building stone, but tens of thousands of residents in Eastern Ontario rely on groundwater from fractures in the limestone rock for their private drinking water supply.  The water is usually hard – there is that calcium carbonate showing up again –  but, in most areas, plentiful.

What is karst?

One of the most fascinating characteristics of the Ottawa Group of rocks, namely the Gull River and Bobcaygeon Formations, is that they form a karst landscape that has been sculpted by natural chemical weathering from infiltrating rain water and migrating groundwater. If you see smooth limestone blocks separated by wide spaces, you have likely found karst. When karst is well established, such as when there are limestone caves and sink-holes in an area, great care has to be taken when going about our regular course of business. Karst can pose a geotechnical hazard since it can produce unstable ground.  It is also very susceptible to land use impacts so it’s groundwater can easily become contaminated.

There are famous and beautiful karst landscapes around the world, from the cenotes in the Yucatan Peninsula to the South China Karst, which is a UNESCO World Heritage Site. Here is an example of a smaller karst landscape in our own backyard.


Unmodified public domain photo by P199 via Wikimedia Commons.

Look for regularly spaced joints in the bedrock of the Burnt Lands Alvar near Almonte.

1 Donaldson, A. and Halfkenny, B., 2011. Stop 3:  Champlain Bridge, Gatineau:  Stromatolite biostrome in Paleozoic strata, in Geological highlights of the National Capital Region; Geological Association of Canada – Mineralogical Association of Canada – Society of Economic Geologists – Society for Geology Applied to Mineral Deposits Joint Annual Meeting, Ottawa 2011, Guidebook to Field Trip 5B, p.6.

2 Williams, D. A., 1991. Paleozoic Geology of the Ottawa-St.Lawrence Lowland, Southern Ontario; Ontario Geological Survey, Open File Report 5770, 292p.

3 Easton, R.M. 1992. Geology, Eastern Ontario, Paleozoic and Mesozoic Geology of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.  915-91

To access the web pages of other display rocks, you can also use the drop down menu on the website’s navigation bar.

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