Building a Sedimentary Wall Layer by Layer

In my job as a Web Content Developer at the Canadian Museum of Nature, I often get an inside look at the work my colleagues are doing. Recently, I happened across a set of photos showing one of our exhibition contractors casting a sedimentary rock face on a remote hill. When I learned more about the casting process and the exhibit, I was impressed and I thought that you may be interested too.

Our contractor, Research Casting International (RCI), is building us a new interactive exhibit. It’s a replica of a remarkable sedimentary rock deposit in Saskatchewan.

The sedimentary rock face in situ.

The colours in this freshly exposed rock face are still vibrant because its moisture has not yet evaporated. This is the face that is reproduced in the museum’s new Vale Earth Gallery. Image: © Research Casting International

RCI’s technique and artistry are marvellous—it’s an utterly convincing reproduction. It looks so good that it’s even difficult to tell where the cast picked up bits of real rock from the mould.

A man stands on the side of a hill holding a shovel.

Pickaxes, shovels and a compressed air wand were used to clear the earth away from the sedimentary rock face to be cast. Image: © Research Casting International

The hard work started on a steep slope of a hill in Saskatchewan. Two days of digging 12 feet (3.5 m) into the hillside revealed the 7′ high × 16′ long (2.1 m × 4.9 m) face of sedimentary rock that would be replicated.

View of the moulding site on the side of the hill.

The rock face that was exposed in the hill is obvious in this photo, thanks to its covering of moulding material. The debris field of the earth that was dug out spills below. A 225-foot-long (68 m) compressor hose runs from the face to a truck at the bottom of the hill. Image: © Research Casting International

Two men paint moulding material on the sedimentary rock face.

Two thirds of the casting crew apply the first layer of moulding material to the rock face. This stage took about three days. The final layer took about half a day to apply. The finished mould was left for a day to dry. Image: © Research Casting International

To make the mould for the cast, the RCI team first applied a skin of latex and coarse cloth to the entire surface of the rock face, taking great care to coat even the finest details. Next came a coat of a different latex to act as a shield between the first layer and the last. The final layer of fibreglass and polyester resin provided a sturdy cradle to support the latex/cloth layer, helping it keep its intricate three-dimensional shape. Thus the mould comprises two parts: the flexible latex layer and the fibreglass shell.

Two men sit on the ground, removing rock from the mould that is lying between them.

Getting the latex layer down the hill to the truck was difficult because it was very heavy: the 200–250 lb. (90–110 kg) of latex brought about 400 lb. (180 kg) of rock and clay with it when it was pulled away from the face. In the photo above left, the crew is removing as much rock as they can before folding it over on itself to try to roll it to the truck below. Image: © Research Casting International

The mould fills the floor of the truck box, and a man stands on top of it.

Fortunately, the mould fit in the truck, even with all the equipment filling the front end of the box. Image: © Research Casting International

Back in the shop in Trenton, Ontario, RCI assembled the mould and laid it on the floor. They built a “bridge” on wheels to go over the mould so that the casting crew could access every part from above, usually while lying down. Two people worked on the bridge for about two days while four others mixed and coloured the casting material and otherwise supported the hands-on work.

The first layer of the cast is the surface that you will see in the gallery. It is a gel coat polyester resin that was tinted to match the real rock. The crew brushed each batch of coloured gel into the mould to match the varied colouration of the real rock face.

Once that was done, the crew built up a thicker layer on top out of fibreglass and polyester resin. That layer forms the back and supports the cast, which is like a wall in the gallery. RCI also incorporated steel fittings into that layer for attaching the wall to the custom steel frame that holds it up.

Once the frame was welded on, what remained to do in the shop was to peel off both parts of the mould, and then the wall was basically ready for installation in our new Vale Earth Gallery.

Three people stand near the wall in the unfinished gallery.

Exhibit developer Nicole Dupuis, lead RCI creator Jeff Haworth and now-retired palaeontologist Steve Cumbaa discuss which fossils to put where on the wall face. Image: Kathleen Quinn © Canadian Museum of Nature

After the wall arrived in the gallery, a museum palaeontologist, the exhibit content developer and the lead RCI creator consulted on the applied fossils. The fossils will serve as traces of events in Earth’s history to be discovered by visitors as they interact with the exhibit. All the fossils represent species that are appropriate to the time and place, and all are casts of real fossils—most of them from the museum’s collection.

A fossil projects from the wall.

When you visit, see if you can spot this cast of a fossil crocodile scute. Image: Kathleen Quinn © Canadian Museum of Nature

As of this week, not much remains to do. Once the last couple of fossils are in place, RCI will apply a final coat over everything to seal and consolidate the surface. To finish, the museum team will complete and install the exhibit labels and guide for helping visitors explore the secrets that lie within.

And that’s the point of the exhibit: we’re inviting visitors to put on their pretend geologist’s hat and “read” the rock face to discover the stories that sedimentary rock records in its layers.

Knowing that the Earth is in constant change, I’m pretty amazed that any record of Earth’s billions of years is preserved so that we can find and interpret it.

Sedimentary rock is able to preserve traces of the environment (think fossils) and geological events (e.g., erosion, tectonic plate movement) because of the way it forms. Sedimentary rock is the result when particles from weathered and eroded rock accumulate in layers and then get compacted and cemented together. The traces are preserved when they get sandwiched in the layers and transformed into rock along with the sedimentary particles.

Illustration of two dinosaurs on a beach, with the meteorite strike in the distance.

Only about 10 seconds after the meteorite impact and already the plume of vaporized rock and water has reached a height of 100 km. The hadrosaurs in the foreground have about 20 minutes before the shock wave kills them. Image: Ely Kish © Canadian Museum of Nature

When the museum team was developing plans for the gallery, they knew they wanted to include something that would let visitors discover such traces—and the stories they embody—for themselves.

Once the team hit upon the idea of having a sedimentary rock face in the gallery, the decision to make it a Canadian example of an extraordinary formation known as the K-Pg boundary soon followed.

This formation is important because scientists were able to interpret the evidence it presents about Earth’s history and figure out that a giant meteorite strike was likely a main cause of the extinction of the dinosaurs.

The K-Pg boundary is a thin layer that settled across the world about 65.5 million years ago when the meteorite strike threw up vast amounts of debris and ash. Its line runs across the middle of the museum’s replica, a mere moment in the approximately six million years represented by the wall’s layers.

The sedimentary rock wall in the gallery.

The K-Pg boundary is distinctly visible as thin white line (ash) on top of a thin dark line (the dust and debris; not discernible here). In the real rock face, the thick dark band below the boundary line is simply staining from coal that leached from the much thicker dark layer above the line. Image: Kathleen Quinn © Canadian Museum of Nature

We hope that visitors to the gallery will enjoy spotting this trace and others in the wall and learning about what they mean—activities that are similar to what palaeontologists and geologists do.

So, on your next visit to the gallery, make sure to look for the large sedimentary rock face that’s quietly minding its own business in the back. Get close and look deeper, and perhaps once your visit is over, you’ll find yourself looking at the rocks around you with new understanding and interest in the history they record.

And of course, before “digging in”, I hope you’ll take a moment to admire the stunning realism of the replica rock face.

The new Vale Earth Gallery opens at the Canadian Museum of Nature on November 30, 2012.

This entry was posted in Exhibitions, Rocks and minerals, Tools of the trade and tagged . Bookmark the permalink.

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