👉 A plack (or a "plaque") is a type of rock or sedimentary material that contains a high concentration of organic matter. Planks are typically found in areas where there has been frequent erosion, such as on riverbeds or in the sedimentary layers of a river. They can be found in various sizes and shapes, ranging from small stones to large boulders. Planks are rich in nutrients, including nitrogen, phosphorus, and other elements that can aid in the