is about 5 m, while nearby kaolin deposits developed in the Mesozoic are much thicker. It has been argued that in the late Neoproterozoic sheet wash was a dominant process of erosion of surface material due to the lack of plants on land. Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts in areas unsuitable for vascular plants. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to intertidal zones and then to land. Lichens, which are symbiotic combinations of a fungus (almost always an ascomycete) and one or more photosynthesizers (green algae or cyanobacteria), are also important colonizers of lifeless environments, and their ability to break down rocks contributes to soil formation where plants cannot survive. The earliest known ascomycete fossils date from in the Silurian. Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose feces are a major source of organic components. Burrows have been found in Ordovician sediments, and are attributed to annelids (worms) or arthropods. Plants and the Late Devonian wood crisis In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land required plants to become internally more complex and specialized: photosynthesis is most efficient at the top; roots extract water and nutrients from the ground; and the intermediate parts support and transport. Spores of land plants resembling liverworts have been found in Middle Ordovician rocks from . Middle Silurian rocks from contain fossils of true plants, including clubmosses such as Baragwanathia; most were under high, and some appear closely related to vascular plants, the group that includes trees. By the Late Devonian , abundant trees such as Archaeopteris bound the soil so firmly that they changed river systems from mostly braided to mostly meandering. This caused the "Late Devonian wood crisis" because: They removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus causing an ice age in the Carboniferous period. This did not repeat in later ecosystems, since the carbon dioxide "locked up" in wood was returned to the atmosphere by decomposition of dead wood, but the earliest fossil evidence of fungi that can