William McMahon and Neil Davies examined stratigraphic data from all of the Earth’s known alluvial [materials left behind on land which has been flooded or where a river once flowed] formations – from 3.5 billion years ago (during the Archean eon) to the Carboniferous Period (approx. 300 million years ago). This veritable deluge of data led to their conclusion that the increase in the proportion of mud retained on land is coeval [“of the same age”] with the evolution of vegetation. In the absence of any factor restraining their movement, these alluvial deposits – of mud, silt and clay – would normally be washed off the land into the sea. Rocks formed at those terrestrial sites, at those times, would therefore have less of those materials in them.
However, from about 460 millions of years ago, something changed and more of this material was retained on land, as evidenced by the increased proportion of alluvial materials contained in such land-formed rocks. That ‘something’ is inferred to be the increasing vegetation cover of the land by plants whose root-like structures, and roots proper, act as filters to retain more of those alluvial materials on land. This is yet another intriguing example of the transformative, terra-forming power of plants, especially the landscaping activities of their roots*.
This present study extends previous work by Neil Davies and Martin Gibling, which made a connection between presence of terrestrial vegetation and enhanced alluvial deposits on land between the Cambrian and Devonian Periods (541 – 359 millions of years ago), by demonstrating that geological periods much earlier than that where terrestrial alluvial deposits were virtually non-existent were also effectively devoid of land plants, thereby emphasizing the vegetation-alluvial connection. And is another example of what’s been described as ‘evolutionary geomorphology’. It’s also a lesson from history of the ability of plants to reduce erosion of land.
Sadly, it’s yet another of those lessons we humans don’t appear to have heeded as we are surprised to note that removal of land-occupying trees – the phenomenon of deforestation – has ecological consequences, not least of which is deposition of silt into streams and waterways (Cecilia Gontijo Leal). Where are those roots when you need them..? Or, Roots, providing the bedrock upon which the Earth’s green mantle is constructed? Discuss…
* For an awesome graphic that graphically demonstrates this phenomenon, see the commentary on the McMahon and Davies paper by Woodward Fisher.