The Grand Canyon’s immense geological layers offer a window into Earth’s deep history. Around 340 million years ago, during the Mississippian Period, this region was profoundly different. It was not a vast gorge, but the floor of a sprawling, shallow sea, teeming with marine life.
A Submerged World
During the Mississippian Period, the Grand Canyon region was largely submerged beneath a vast, warm, shallow tropical sea. This ancient marine environment extended far inland across North America, indicating significantly higher global sea levels. The water in this expansive sea was relatively clear.
The depth of this sea rarely exceeded 300 feet, allowing sunlight to penetrate the seafloor. Marine sediments accumulated, forming rock layers hundreds of feet thick as the seafloor slowly subsided. This period was characterized by a warm, greenhouse climate, with the North American continent positioned near the equator.
Life in the Ancient Sea
The warm, clear waters of this ancient sea supported a diverse array of marine life. Among the most prevalent organisms were crinoids, often referred to as “sea lilies” due to their plant-like appearance. These filter feeders formed extensive “meadows” on the seafloor. So abundant were crinoids that the Mississippian Period is sometimes known as the “Age of Crinoids.”
Other common invertebrates included brachiopods, which are shelled organisms resembling clams, as well as various types of corals. Bryozoans and foraminifera also flourished in these shallow marine conditions. While trilobites were in decline, early forms of sharks and bony fishes also inhabited these waters.
The Formation of Redwall Limestone
The abundant marine life and specific environmental conditions of 340 million years ago were instrumental in forming a distinctive rock layer known as the Redwall Limestone. This thick layer is primarily composed of calcium carbonate derived from the shells and skeletal remains of countless marine organisms. As these sea creatures died, their shells and fragments accumulated on the seafloor as lime mud.
Over millions of years, this limy material underwent compaction and cementation, transforming into the solid rock we recognize as limestone. While the Redwall Limestone itself is inherently a bluish-gray color, its prominent red appearance in the Grand Canyon is a result of iron oxides. Iron-rich sediments from overlying rock layers bled down over time, staining the surface of the Redwall Limestone with its characteristic reddish hue.
Uncovering the Past
Scientists piece together the Grand Canyon’s deep history through various geological and paleontological methods. Paleontologists study the abundant fossils embedded within the Redwall Limestone, which serve as direct evidence of the marine organisms that once lived there. These fossilized remains offer insights into the ancient environment and the types of life forms that thrived in that period.
Geologists employ stratigraphy to understand the sequence of events and environments that formed the canyon’s strata. By analyzing the composition and structure of sedimentary rocks like the Redwall Limestone, scientists can interpret the conditions under which they were deposited. Geological dating techniques allow scientists to determine the absolute ages of these rock layers, providing a precise timeline for the Grand Canyon’s distant past.