A Recipe for Rock Spheres
Scattered across the badlands of southeastern Alberta are thousands of reddish, spherical boulders. Many are massive, with some measuring up to 2.5 meters (8.2 feet) in diameter, and are considered among the largest of their kind in the world. These are not alien artifacts or dinosaur eggs, but geological formations called concretions. Their story begins around 75 million years ago, during the Late Cretaceous period, when a vast inland sea called the Western Interior Seaway covered this region.
The geologic unit they are found in is the Bearpaw Formation, a thick layer of marine shale and sandstone. The process started when some sort of nucleus—perhaps a shell, a bone fragment, a leaf, or other organic debris—settled into the soft seafloor mud. Minerals dissolved in the seawater, primarily calcite and iron oxides, began to precipitate out of the water and cement the sand and silt particles around this central point. Over immense spans of time, more and more layers of this mineral cement were added, growing outward from the nucleus in a nearly perfect sphere. The process is like how a pearl forms inside an oyster, but on a much grander scale.
Time's Slow Reveal
For millions of years, the concretions remained buried within the layers of the Bearpaw Formation. The mineral cement that binds them makes them far harder and more resistant to erosion than the surrounding shale and mudstone. As geological forces lifted the ancient seabed and the last Ice Age ended, meltwater rivers and incessant prairie winds began to carve away the softer rock. This slow, persistent erosion exposed the durable concretions, leaving them scattered across the surface of the coulees scattered across the surface.
Many of the concretions are a distinct reddish color, the result of iron oxides within the sandstone essentially rusting over time. While many are solid sandstone, some have been broken open by the elements, revealing their layered internal structure, like the growth rings of a tree. A special type, known as a septarian concretion, features a network of internal cracks. These cracks, or septaria, likely formed due to shrinkage of the concretion's core as it dehydrated. Later, these cracks filled with other minerals, often yellow calcite, creating complex internal patterns. Within the Bearpaw Formation, concretions sometimes contain exceptionally well-preserved fossils, including ammonites like Placenticeras and Baculites, bivalves, and even the remains of marine reptiles like mosasaurs.