What is the black substance in fern imprints?

The black substance in fern imprints is typically carbon, which results from the decomposition of organic material over time. When ferns and other plants die, their remains can become buried and subjected to heat and pressure, leading to fossilization. This process can create impressions or imprints of the plants in sedimentary rock, where the carbon content remains visible as dark marks. These imprints provide valuable insights into the types of vegetation that existed in prehistoric environments.