Look for an abundance of fossils in display rocks 13, 15, and 16.


Photo by Brent Eades.

Display rock 13 is a block of limestone from the Cavanagh quarry at the Burnt Lands east of Almonte.

How did this rock form?

During the Ordovician Period, our part of the world was close to the equator.  The area north of the tropics was almost entirely ocean, and most of the world’s land was collected into the southern supercontinent Gondwana.

The shallow seas that covered our region hosted a rich ecosystem of marine life.  Preserved in the limestone of eastern Ontario is a variety of  barrier-front, barrier-top and lagoon environments. Clam-like brachiopods and spiral-shaped gastropods lived on the nutrient-rich sea floor, along with plant-like crinoids on long stems. Colonial tabulate ‘honeycomb’ corals were some of the most prolific reef-builders in Earth’s history. Colonial bryozoa, which first appear in the early Ordovician fossil record, were also abundant.  The dominant predators were squid-like cephalopods with cone-shaped shells.  When these ocean critters died, their remains were covered by accumulations of lime-rich sediments.  Over time their hard bits were preserved as fossils in limestone, a rock rich in calcium carbonate, CaCO3.


Image © The Manitoba Museum, Winnipeg, MB.

On display at the Manitoba Museum is a diorama of an ancient seafloor environment during the Ordovician.  Fossils of these marine animals are found in the limestone and shale of the Almonte region.

Rugose Corals

Photo by Asia Reid.

Look for these and similar fossil fragments of rugose coral on the surface of display rocks 13 and 15.

Public domain image by Mark A. Wilson via Wikimedia Commons.

Side and top views of Ordovician age horn corals. 

Rugose corals are an extinct group of solitary and colonial corals that were abundant on Ordovician seafloors.  They’re also known as horn corals because they have a horn-shaped chamber with a wrinkled, or rugose, wall.  Fossil fragments can be found in limestone outcrops across the Almonte region.

Tabulate corals

Tabulate corals were colonial organisms.  The tiny polyps, the actual organisms, secreted an exoskeleton of calcite to create a colony made up of corallites — small, closely packed cells or tubes, similar in appearance to a honeycomb. When viewed from above in cross section, the corallites are hexagonal in shape, like the ones in the photo below. Tabulate corals emerged during the Ordovician and became important reef builders during the Silurian and Devonian, before going extinct during the Permian Period, about 245 million years ago.


Photo by Asia Reid.

Fossilized honeycomb coral in limestone.  Look for other colonial coral fossils too in display rocks 13 and 15.

Photo courtesy Metcalfe Geoheritage Park Committee.

This sample of fossil honeycomb coral was collected from a roadside outcrop of limestone in the Almonte region. 


Bryozoa was another colonial animal of the Ordovician seafloor.  Like coral, they were filter feeders with colonial structures of calcareous exoskeletons, but much smaller in size.  Bryozoa fossils are common in the limestone of the Almonte region as branch-like structures, or as small, rounded mounds.

Photo by Asia Reid.

A red line was added to this photo as an outline of the branched structure of a fossilized bryozoa colony.  Look for similar fossils of bryozoa on display rocks 13 and 15.

Photo courtesy Metcalfe Geoheritage Park Committee.

Close-up view of a small colonial mound of fossilized Ordovician bryozoa from the Almonte region.


Crinoids during the Ordovician were marine animals on long stalks that resembled a plant. Rooted in the ocean floor sediment, these animals would filter feed as the water flowed past them.  When Crinoids died, they’re segmented stalks fell apart into small cheerio-shaped columnals.  Studies of crinoid fossils have show they are commonly associated with sandy shallow areas of a lagoon or shoal.

Modern crinoids are commonly known as feather stars, and are typically stalkless, allowing them to move through the water rather than being rooted in one place. Stalked varieties do still exist, but are very rare in modern oceans.

Public domain image by William I. Ausich via via Wikimedia Commons.

Anatomy of a crinoid. 

Image courtesy Metcalfe Geoheritage Park Committee.

Crinoid ossicle with star-shaped centre.  

Image courtesy Metcalfe Geoheritage Park Committee.

Columnal crinoid fossil from the Almonte region. 


Clam-like, bivalve brachiopods first appeared in the Cambrian Period before tropical seas covered the Almonte region.  During the Paleozoic Era they were the most abundant filter feeders and reef builders of the world’s oceans.  Although mostly extinct, some brachiopod species live in marine habitats today.

Image by Asia Reid.

Fossil brachiopod (green outline) and gastropod (red outline) on display rock 13.  Look for similar fossils on display rock 15 too.  

Photo courtesy Metcalfe Geoheritage Park Committee.

Close up view of a brachiopod fossil from the Almonte region.


Marine snails with spiral-shaped shells were common on the seafloor in our area during the Ordovician Period.  Their fossil remains can be found in the limestone bedrock all across the region east of Almonte.  They range in size from small to hand-width.

Photo courtesy Metcalfe Geoheritage Park Committee.

Gastropod fossil from the Almonte region.


Squid-like cephalopods with long straight shells (orthocones) were the dominant predators of Ordovician oceans.  About 450 million years ago the largest animal in the world was a jet-propelled cone with tentacles.  Orthocone fossils in the Almonte region range in size from a few cm to more than 2 m long.

Public domain image by Archbob via Wikimedia Commons.

Artist’s interpretation of Ordovician orthocone cephalopods.

Photo by Asia Reid.

Cross section view of a small orthocone cephalopod in limestone showing the internal chambers of the cone-shaped shell.  Look for others in display rocks 13 and 15. 

Image courtesy Metcalfe Geoheritage Park Committee.

Section of a cone-shaped orthocone fossil from the Almonte area.


Trilobites were hard-shelled arthropods with multiple body segments.  They thrived during the Cambrian and Ordovician Periods, but disappeared in the mass extinction of the Permian, about 245 million years ago.

Public domain image by San Gon III via Wikimedia Commons.

Trilobite anatomy.

Trilobite thorax sections.    Image courtesy Metcalfe Geoheritage Park Committee.
Underside of a trilobite pygidium.    Image courtesy Metcalfe Geoheritage Park Committee.

These and other fragments of trilobite fossils have been found at limestone outcrops across the Almonte region.

Image courtesy Metcalfe Geoheritage Park Committee.

Mold and cast trilobite fossil in shale from the Almonte region.

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

Share this: