Petrified Wood: Preserving Alberta’s Natural and Human History

Designated as Alberta’s official stone in 1977, petrified wood is part of the geology, palaeontology and archaeology of the province. Petro (meaning ‘stone’ in Greek) is the root word of petrifaction, the process whereby an organism is mineralized or turned to stone. Petrified wood is a 3-D fossil that can appear like modern wood at first glance.

Figure 1. Examples of raw petrified wood cobbles and pebble from Alberta.

 

Petrifaction of wood involves two processes: permineralization and replacement. Both of these processes take place at the same time and involve the flow of groundwater, rich in dissolved minerals, through the wood. During permineralization, the internal and external features are surrounded and encased by minerals that have crystallized out of solution. During replacement, decomposing plant tissues are gradually substituted with minerals, which replicate the original structure in remarkable detail. Depending on the chemical composition of the groundwater, silica, calcite, pyrite and a number of other minerals can petrify wood. Quartz and other varieties of silica (SiO2) are the most common minerals: these fossils are said to be silicified.

Wood grains and tree rings can be preserved in the rock and are the most useful features for identifying a specimen as petrified wood. Without these remnant features, petrified wood can be mistaken for a number of silica-rich rocks found in Alberta. Many varieties of chert, flint, and other silicified organic material look similar and can even exhibit banding that closely resembles tree rings. The colour of petrified wood depends on the composition of the minerals present. Pure quartz, for example, is colourless but can be a variety of colours when small amounts of certain elements are included in its crystal structure. Weathering can also affect the colour of petrified wood, sometimes appearing bleached due to prolonged sun exposure.

 

Figure 2. The top image is of a polished cross-section (cut) through petrified wood. The bottom eight images are microscope photographs showing the variability of petrified wood in Alberta. Some petrified wood is so thoroughly silicified that it is smooth and uniform in composition while other specimens are platy and retain the layers (‘laminae’) of the original wood grains.

 

Petrified Wood in Alberta

Alberta is world famous for the incredible dinosaur fossils that have been found in places like Dinosaur Provincial Park, but the badlands of Alberta are also home to some spectacular plant fossils that lived alongside dinosaurs. Petrified wood from the Cretaceous period, 65 to 145 million years ago, is one such type of fossil. Palaeobotanists, people who study ancient plants, have used well-preserved pieces of petrified wood to gain information about past environments by looking at thin, cross-sectional slices under a microscope. Similar to how people study modern trees, the species and cross-sectional growth patterns of petrified wood can give us information about the climate of a region when the tree was alive. Ancient conifer trees that are similar to modern pine, cedar and redwoods are some of the species that have been identified in Dinosaur Provincial Park. This kind of research continues to uncover more information about ecosystems of the past and provides context for other organisms, like dinosaurs, that lived in Alberta at this time.

Figure 3. The badlands of Dinosaur Provincial Park have yielded petrified wood from the age of dinosaurs (photograph courtesy of Travel Alberta and Jeremy Klager).

Throughout other areas of Alberta, eroded pieces of petrified wood are commonly found in gravel pits and along river banks. Because these pieces have been removed from their original position in the sediment and are mixed with rocks of different ages, it can be challenging to know how old they are. Most often they are no bigger than cobbles (less than 30 cm in diameter). However, the largest piece ever found in Alberta was a tree stump measuring 2 m across and 88 cm tall at its highest point. A local guide on the Athabasca River reported the find to authorities. Staff of the Royal Alberta Museum (RAM) and Royal Tyrrell Museum recovered the stump in 2012. They used straps, slings and winches to lift the 3034 kg trunk from the river bank to a wooden barge. The stump was then towed downriver and loaded onto a truck for transport to the RAM, where it is currently housed. Wood grains and tree rings are clearly preserved in the fossil, which can be rare for pieces that have been eroded and reworked by rivers. Experts determined this exceptional specimen to be from the period just after dinosaurs became extinct, 60 to 65 million years ago.

Figure 4. A large petrified wood stump recovered from Athabasca River in 2012 (images courtesy of the Royal Alberta Museum).

 

For the last 13,000 years, petrified wood that is millions of years old has been collected and used by people to make stone tools. Beautiful projectile points (the tips of arrows, atlatl darts, and spears) with visible wood grains have been recovered from Alberta’s archaeological record, along with other lithic tools, cores and flakes. The widespread distribution of recorded archaeological sites in Alberta, where petrified wood has been recovered in the form of an artifact used by pre-contact people, is shown on the map below.

Figure 5. Projectile points from southern Alberta made of petrified wood. The top panel displays the most common appearances of petrified wood points while the bottom panel showcases some of the variability.

 

Figure 6. Archaeological sites with pre-contact artifacts made of petrified wood. Petrified wood was collected by pre-contact people in the province for 13,000 years.

 

The highest quality raw materials for flint knapping (making tools out of stone) are typically those that are uniform and fine-grained so they break in a predictable way, such as many quartzites and cherts. Modern flint knappers have noted that the uneven internal structure of petrified wood can make flaking more difficult, but the layers or “laminations” can occasionally help break it into thin pieces or slabs that are ideal for lithic tool production. Although not of the highest quality, petrified wood is still considered to be a consistently reliable material that would have been easily identified on the landscape. It is also found almost everywhere in Alberta, which likely accounts for its ubiquitous pre-contact use in the province.

Collecting in Alberta

Many Albertans have a piece of petrified wood as part of their personal rock collection. Like any other archaeological and palaeontological find in Alberta, the guidelines for collecting are outlined in the Historical Resources Act. Petrified wood cannot be removed from a provincial park, national park or designated protected area. Excavating for petrified wood is also not allowed, which involves digging up a piece buried in the ground or dislodging it from a rock face. Such excavations require a permit and are carried out by professional palaeontologists and archaeologists.

Surface collection, however, where petrified wood is clearly visible on the surface of the ground, is permitted on public lands. If you live in Alberta, you can keep a surface find but you may not sell, alter or remove the specimen from the province without approval from the Government of Alberta. A petrified wood projectile point or other lithic artifact is considered an archaeological find and must be reported regardless of whether it was found on the surface or in the ground.

Petrified wood is deeply engrained in the story of Alberta, representing millions of years of natural history and millennia of human history. By connecting the geology, palaeontology and archaeology of the province, petrified wood is an interestingly complex piece of heritage that is deserving of its title as Alberta’s official stone.

Written by: Emily Moffat, Regulatory Approvals Coordinator, Archaeological Survey of Alberta

Thank you to the Royal Alberta Museum for images of the petrified wood stump, Dale Fisher for photographs of projectile points and Sean Lynch for providing insight into flint knapping petrified wood. Also thank you to Todd Kristensen for providing images and feedback.

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