Mud rocks increased in riverbeds as rootless plants spread around 458 million years ago
Early plants made Earth muddier. Ancient riverbed deposits of mud rock — rocks containing bits of clay and silt smaller than grains of sand — began increasing around 458 million years ago, around the time that rootless plants became common across Earth, researchers say.
Anecdotally, geologists have long noted that early sediment deposits became muddier at some point, and suggested a connection with plants (SN: 6/22/74, p. 398). But no one had ever pinpointed when that muddening happened.
So geologists William McMahon and Neil Davies, both of the University of Cambridge, decided to look for when amounts of mud rock began increasing in 704 ancient river deposits from 3.5 billion to 300 million years ago. The researchers searched through nearly 1,200 published papers for data on mud rock in river deposits, and collected new field data at 125 ancient river outcrops. At