Trees that don't know they're supposed to fall over
Along the coast of Nova Scotia, the world's highest tides relentlessly erode the Joggins Fossil Cliffs, a 15-kilometer-long exposure of rock. This erosion continuously reveals a world from the Pennsylvanian subperiod of the Carboniferous Period, roughly 318 to 303 million years ago. The cliffs are a UNESCO World Heritage site, famous for an unusual phenomenon: entire fossilized trees found standing vertically, cutting through many layers of sedimentary rock. These are known as polystrate fossils.
The ancient "trees" are not like modern ones. They are primarily extinct lycopsids, such as Lepidodendron and Sigillaria, and the giant horsetail-relative Calamites. During the Carboniferous, this area was a tropical swamp near the equator, and these plants grew to heights of over 20 meters. The geological record at Joggins shows that this swampy coastal plain was subject to frequent, massive flooding events. These floods deposited huge quantities of sand and mud very quickly, burying the forest floor and the bases of the standing trees. The tops of the plants would eventually rot and fall, but their sediment-filled stumps were preserved in their original growth positions. This cycle of rapid burial happened repeatedly, preserving multiple generations of forests on top of one another.
The Coal Age record
The interpretation of these upright fossils became a central point of discussion in 19th-century geology. Geologist Charles Lyell, an important figure in establishing the concept of "deep time," visited Joggins in 1842 and 1852. He, along with Canadian geologist Sir William Dawson, saw the polystrate trees as clear evidence of periodic rapid burials within a slowly subsiding basin over immense timescales—not as evidence for a single, global flood. Their work at the site provided information that even influenced Charles Darwin.
The hollow, buried stumps of lycopsid trees also acted as natural pitfall traps. In 1852, Lyell and Dawson made a landmark discovery inside one of these fossilized stumps: the skeleton of Hylonomus lyelli, the earliest known reptile. This small, lizard-like animal, about 20 centimeters long, is the first known vertebrate to have fully adapted to life on land, thanks to the evolution of the amniotic egg. The discovery of Hylonomus confirmed that reptiles existed during the "Coal Age," much earlier than previously thought, and established the Joggins cliffs as a site of global scientific importance. Other fossils found trapped in the trees include amphibians, millipedes, and some of the earliest land snails.