For the Love of Limestone

Geology is hard. Really attempting to comprehend the scale of any geological process requires an intellectual breach that challenges us to reconcile our perception of the relatively static world we observe with formations that were created an incredibly long time ago over an enormous number of years that are effectively impossible to imagine. It’s like trying to picture either the number of stars in a galaxy or molecules in a glass of water – but for me, limestone is even more difficult still.

Limestone is an organic sedimentary rock; meaning that is is mostly made up of the remains of living things that lived, died, accumulated and were buried deep and long enough to become lithified (compressed by the pressure of the weight of the rock and sediments above it into solid rock, basically). Coal is one of the other more common examples of an organic sedimentary rock, which (as we remember from grade school) is composed mostly of plants that accumulated in swampy forest environments and were buried, compressed and lithified deep within the Earth. In the case of limestone it is the skeletal remains of living creatures that lived and died in a long ago oceanic environment that make up the bulk of this rock type. Further still, limestone and other carbonate rock dominant (karst) environments cover about 15 percent of the Earth’s land mass and are distributed widely, making it an absolutley worldwide phenomenon. So not only is that 500 foot tall cliff exposure you are looking at almost entirely made up of dead marine animal skeletal fragments, but similar rock types occur globally.

Fossil Brachiopod embedded in limestone in eastern Arizona

The world in which these animals lived was a very different place. All the continents were either connected together or very close to each other in this warmer, more humid and oxygen rich environment. Tropical seas flooded large swaths of the continents, creating the warm, shallow seas (picture the Bahamas or Australia’s Great Barrier Reef) in which these creatures lived – corals, crinoids, brachiopods, bivalves and tiny foraminifera were some of the most common of these animals.

Crinoid stem fragments in Grand Canyon National Park

Criniod (also called water lilies) have the general morphology of a flower – a foot attaches to the sea floor and a stem made up of small circular plates leads to what looks like a flower at the top where most of the animals vital parts reside, including arms that look like flower petals that are used to grab food. Though still living today, crinoids were much more abundant in these ancient oceans.

Fossil crinoid calyx (the bud and flower at the top) surrounded by other crinoid fragments. This individual is frozen in time with its arms folded back, exposing the mouth (in the center) and its butt (the dark circle off-center above and left of the mouth), understandably making this my proudest fossil discovery to date. Imagine if we were built this way – mouth so close to the butt…

Redwall limestone narrows in Grand Canyon

Probably the most impressive exposure of limestone in Arizona is the Redwall limestone in Grand Canyon National Park, which can be seen throughout much of the park with exposures up to 500 feet tall and stretching along the canyon as far the eye can see and beyond. The Redwall also forms the majority of the scenic, technical slot canyons in the park, making a visit to this stratigraphic unit the ultimate goal of most canyoneering trips in Grand Canyon. Of course the Redwall isn’t really red – the coloration is a stain from the overlying Supai and Hermit layers as water from precipitation carries fine particles from these layers down the cliffs. Where water has carved a narrow gorge in the limestone is where its true color is exposed as a beautiful silver-grey.

The sediments of Redwall limestone were deposited around 340 million years ago in the Mississippian subperiod, in the early Caboniferous period. Monsters inhabit this world, but dinosaurs won’t evolve for roughly another 100 million years. Instead of dinosaurs, the seas are terrorized by giant sea scorpions up to 7.5 feet long and seven-foot long millipedes roam the primitive forest floor. Vertebrates are just getting a foothold on dry land and life makes a great leap forward towards terrestrial domination with the evolution of eggs with a hard, mineralized shell for protection. Most of the world’s coal deposits are created by those swampy forests during this period as well.

Daniel Elson rappelling into the Redwall narrows of Cove Canyon – GCNP

Trying to imagine the number of years and lives that make up an exposure of limestone is difficult enough, but the vastness is complicated even further when you consider just how little of it we can even see. The great majority of Redwall limestone is still buried under its overlying stratigraphic layers, only to be exposed as the canyon is eroded further and deeper. What will the world look like then, hundreds of millions of years into the future? Will the Colorado River basin be rendered into low-lying hills instead of the towering vertical cliffs of the very Grand Canyon? How will life respond to the changing climate and continued shifting of tectonic plates as they march towards their likely future reunification? Will our species even prosper that far into the new, ever-changing world?

Every rock has a story to tell – rich with the marks of a previous world. Limestone is special in that it is made from the very living things of this lost world, laid bare in what they have left behind. Ponder the significance of this unassuming grey stone as you traverse a water-carved hallway of history, hike through a 500-foot deep tombstone, a monument to the power of time and to the utterly countless lives that helped make this world the marvel that it is.

Pat Winstanley

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