• Massive petrified redwood stumps

    Florissant Fossil Beds

    National Monument Colorado

Geology (Unit 2) - The Sedimentary Environment

The Sedimentary Environment

After sediments decompose due to weathering, they are displaced by erosive agents, such as water, ice, gravity, or living organisms Sediments can also be transported in volcanic ash clouds or lahars.

Sediments will settle when the velocity of the transporting medium is no longer sufficient to move the materials. For example, as the velocity of water slows, the sediments will settle out of suspension and be deposited, heaviest particles first. Deposition can occur anywhere along a stream or a river when the velocity of water subsides, or when the stream enters a pond, lake, or the ocean.

Lacustrine Environments

Lacustrine environments exist where the flow of a stream or river is impeded by a physical barrier such as a dam, or when the surrounding topography creates an almost flat surface that is poorly drained, such as a marsh or swamp. Natural dams are sometimes created by glaciers, rockslides, or volcanic lava or debris flows that block valleys through which rivers or streams flow.

Multiple volcanic eruptions and debris flows from the Guffey volcanic complex are thought to have formed the dam that blocked the flow of the stream moving through the ancient Florissant Valley, resulting in episodes of lake formation.

Diagnostic features of lacustrine deposits

  • Limited extent, often circular, elongate, or closely following the topography of the land.
  • Sequences of sedimentary deposits that start coarse and end in finer sediments like shales.
  • Finely laminated mudstones.
  • Fossils of freshwater aquatic invertebrates such as certain freshwater diatoms, gastropods, and fish.

Geological Maps

Did You Know?

illustration of rhinocerous-like mammal

Fossil remains of a brontothere, a rhinoceros-like creature, have been found in the Florissant fossil beds. Brontotheres were enormous, with some standing over eight feet tall.