Geological records reveal that mudrocks emerged around roughly the same time as plants did, 500 million years ago, a new study reports. The results hold important insights for understanding Earth processes, such as erosion, and may even provide new insights into ancient ocean chemistry. Within the geological record, deposits of clay, silt, sand, and gravel left by flowing streams in a river valley or delta create a timeline of processes on Earth. Evidence of mudrock within the geological record is thought to be rare before the emergence of terrestrial plants roughly 500 million years ago, but this notion has not yet been confirmed. Here, William McMahon and Neil Davies analyzed sediment data from nearly 1,200 published reports, as well as from 125 original field investigations. Their results show that mudrock is barely present in sediments deposited during the first approximately 3 billion years of Earth's sedimentary rock record, yet it is common or even dominant after the middle Paleozoic era. The presence of mudrock increases steadily over time, the authors say, which rules out the possibility that it is a result of cyclical or episodic phenomena, such as tectonic or climatic controls. The steady increase of mudrock coincident with the emergence of plants strongly suggests a relationship between the two events. Woodward Fischer provides more context in a related Perspective, noting that, "If the timing of the initial increase in mudrock is correct, it suggests that the earliest plants made a substantial impression on the landscape."