Photo by Chris Collom. Leisegang rings leisegang rings Leisegang rings are concentric bands of alternating coloration, typically found in sandstones, and typically with an orange, red, or brown colour. The water and oxidation processes diffuse inward along a discrete front, whose position varies depending upon the water and oxygen supply. In outcrop, concentric leisegang rings occurred around every fracture in the sandstone cliff this specimen was obtained from. Cretaceous, from the Rocky Mountain foothills in Alberta.
Concretions are zones of cementation formed in sedimentary rocks.
Concretions can have a wide variety of shapes that are determined by the chemistry of mineral solutions in the sediment, variations in porosity and permeability of the sediment, and often variations in chemistry caused by the decay of animals or plants. In this example, the concretion is siderite-cemented shale. Ellipsoidal shapes were most common. As the conretion was weathered, the alteration of the siderite to iron oxides penetrated from the outside towards the interior, mainly as a result of the incremental infiltration of water. This process produced leisegang rings like the previous example.