Nov 11, 2018 at 2:00 AM
We tend to think of major features such as mountains and rivers as timeless — and on human time scales they are — but they have beginnings. A recent bit of work published in the journal Cretaceous Research gave some fascinating data on the origin of America’s greatest river, the Mississippi.
So how does one date the origin of a river? By the sediment it carries and then dumps.
Study of the McNairy Sandstone in southernmost Illinois shows that late in the Cretaceous Period, around 70 million years ago, near the end of the reign of the dinosaurs, sediment derived from rocks in Canada and the Appalachians began arriving in the upper reaches of what would eventually become the Mississippi delta.
We know it came from those places thanks to an incredible mineral, zircon, or zirconium silicate, a minor accessory mineral in many igneous rocks such as granite. Usually light brown with a brilliant luster, red, orange or yellow crystals of zircon are known as the gemstone hyacinth. Two things make small, essentially microscopic, crystals of zircon valuable for geological research.
First, they are tough — they can be exposed, eroded, transported and deposited multiple times without losing their integrity. Second, when they crystallize they incorporate small amounts of uranium, which then decays at a known rate and can be used to determine when the crystals first formed.
The Canadian Shield, that large area of ancient rock at the core of the continent, is composed of about a dozen parts, each of which might once have been a separate microcontinent. Those parts, called provinces, formed at different times and subsequently became welded together. For example, the Superior Province formed about 2.5 billion years ago, the Penokean at 1.85 billion, and the Grenville at around 1.4 billion. The ages are based on their zircons.
The McNairy Sandstone contains tiny zircon crystals of exactly those ages. Although 74 percent came from the Grenville Province, about 10 percent came from the Appalachians, showing that this was a major river system.
So why did the river start then? The Gulf of Mexico was rapidly opening as the most recent supercontinent, Pangaea, was breaking apart to form the continents we know today. As the separation progressed, the land bordering the Gulf subsided and water began flowing toward the Gulf, initiating the mighty Mississippi.
CBS journalist Charles Kuralt once said, “The Mississippi River carries the mud of thirty states and two provinces 2,000 miles south to the delta and deposits 500 million tons of it there every year. The business of the Mississippi, which it will accomplish in time, is methodically to transport all of Illinois to the Gulf of Mexico.”
Although talking about a different river, perhaps Rudyard Kipling said it best in his poem “The River’s Tale”, which begins:
“Twenty bridges from Tower to Kew —
Wanted to know what the River knew,
Twenty Bridges or twenty-two,
For they were young, and the Thames was old
And this is the tale that River told:”
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.