Nearly two billion years of Earth’s history are contained in the layers of rocks in the Grand Canyon. Gazing down into this vast chasm I had occasion to ponder the timescale of the canyon and of the planet. To better understand that timescale, to understand the changes that are recorded in those rocks dating back nearly two billion years, I dived into the geology.
I am no expert in geology, and in fact pretty much only know what I learned at the Grand Canyon and from a little background research to provide context into what I read at the park. I had a great time learning the little I did, so I want to share what I discovered with those of you who also don’t know anything about rocks and the stories they contain, but are interested to read a little about it. (And if you are not interested in reading about rocks, that’s okay, come back in a couple days for Grand Canyon Part 3, where I share our hike down to the rim and a picture of the cutest squirrel you’ve ever seen.)
The “Trail of Time” is an interpretive trail that starts at the Yavapai Museum of Geology. It is actually a timeline, where a long stride represents a million years. A sample of each rock layer in the canyon wall is located along the timeline when it was formed. The rocks in the Grand Canyon are 270 Million years to 1.8 Billion years old. The last stone sample from the “basement” rocks of the canyon is located before the Verkamp’s Visitor Center, but the timeline continues along the rim trail for another 1.7 miles to the beginning of Earth 4.560 billion years ago. This is a brilliant idea for an interpretive trail. It turns the abstract idea of hundreds of millions of years into a physical representation, and the stone samples are there to touch, so you also feel the textures of the different types of rocks.
You probably learned some basics about rocks as a kid, but I will review them in case you forgot. There are three basic types of rock: sedimentary, igneous and metamorphic. Sedimentary rock, as the name suggests, is rock built up of sediment such as sand or mud; igneous rock is hardened magma; metamorphic rock starts as either sedimentary or igneous and is transformed by heat and pressure into a much harder rock. Minerals cycle through these different rock phases. Rock of any kind that is exposed to the surface is broken down by wind and water into small pieces that become sediment. Deep underground, rock melts into magma that hardens into igneous rock when it reaches the cooler earth surface. Surface rock gets pulled deep underground by tectonic movement where heat and pressure transform it into metamorphic rock. Fossils can only be found in sedimentary rocks because they form when plant or animal remains are trapped in the sediment.
The lower rock layers of the Grand Canyon are metamorphic rock, but most of the layers are sedimentary rock. Based on the type of sedimentary rock, geologists can determine what the environment covered the land when the layer formed. Limestone forms in shallow seas, sandstone signifies beach or sand dunes and shale is formed from mud in swamps and coastal plains. So the rock layers in the Grand Canyon are like a history book of the land over millions of years. This history book is missing some chapters where the rock layers were eroded away. Layers formed over millions of years were subsequently completely eroded over time, leaving no trace of what the land was like for these long periods. Gaps such as these in the geological history recorded in stone are called “unconformities”.
Sedimentary rocks built up two miles deep as seas advanced and retreated over the land of the Grand Canyon many times over the nearly two billion years. At the bottom lay the Vishnu Basement rocks, metamorphic and igneous rocks formed 1.84-1.68 billion years ago. After a four million year gap, the Grand Canyon Supergroup formed 1,250-650 million years ago. These mostly limestone and sandstone layers are tilted from tectonic movement following their deposition. Once the layers were pushed up, erosion leveled the land leaving a bedrock of angled layers under a flat surface. Sediment continued building up the land in flat layers over this tilted understory comprising the Supergroup. The angled orientation relative to the surrounding layers is called an “Angular Unconformity”. (Go back to the first picture and look for the angled layers at the bottom to see what I am talking about).
The next flat layer was deposited 524 million years ago (126 million year gap), by this time multicellular animal life had formed as evidenced by the fossil record. Other, smaller gaps exist in the timeline where erosion erased the geological record over time, as you may notice if you look closely at the pictures of all the samples below. If you happen to be interested in reading even more about Grand Canyon geology, I came across this well-researched blog entry while writing this post.
Sixty million years ago after two miles of sedimentary rock had built up, the Colorado Plateau was uplifted and compressed from distant tectonic plate collisions. The top mile of rock layers eroded, so the youngest rocks in the Grand Canyon are the 270 million year old Kaibab formation. Dinosaurs first appeared 231 million years ago, so the rocks in the Grand Canyon are too old to contain dinosaur fossils; sediment layers formed when dinosaurs roamed the earth were lost to erosion. The top-most (hence youngest) layer formed during the Permian Period, when the land was a tropical sea. Sea creature fossils are abundant in this layer of limestone.
Uplift of the Colorado Plateau made the carving of the Grand Canyon possible. Most canyon experts believe that the Colorado river carved the Grand Canyon over the last 6 million years, but apparently this is a source of debate, with some geologists believing the canyon is much older.
Compared to these geological timescales, the human cultural history in the area is miniscule. archeological evidence suggests that nomadic hunters lived near the canyon at least 10,000 years ago. Ancient Puebloans farmed the canyon bottom 1,000 years ago. Americans started exploring the canyon about 150 years ago and began capitalizing on tourism to see the natural wonder just over 100 years ago.
On our final day at the canyon, I attended a ranger-led walk about fossils. As it turned out, Texas public schools were on spring break by this time and the walk was filled with young children fulfilling the “Junior Ranger” requirements. I want to share two observations from that program that have nothing to do with fossils or geology. At the beginning of the program, the ranger asked the group what we thought the most “vicious” animals in the canyon are. The answer she was looking for is rock squirrels, because people feed them and sometimes they bite and scratch (and carry plague, according to the warning signs). I understand that she was trying to make the point that you shouldn’t feed wild animals, but teaching children that the animals are “vicious” is the wrong message to send. The park should be teaching respect for the animals, and the dangers that wildlife face when they learn to depend on and to approach humans for food. The other non-fossil observation from this program is an early start to the underrepresentation of women in science. At one point in the program the ranger asked for volunteers. A vast majority of the children in attendance were girls, maybe a 20 girls/4 boy ratio, yet all three volunteers were boys. I wasn’t paying close enough attention to see if the ranger had overlooked girls raising their hands, or if only the boys stepped forward, but the discrepancy is bothersome.
Enough of my opinions about how the program was ran, and onto the fossils! As mentioned earlier, the rocks in the Grand Canyon are too old to contain those most famous of fossils, dinosaur fossils, but there are sea creature fossils (seashells are good at fossilizing) and even footprint fossils. If you scroll down to the Coconino Sandstone, the third from the bottom in the pictures of samples below, you will see fossil footprints from a reptile traversing sand dunes during the Permian Period. The ranger fossil program stayed on the rim and examined a fossil bed in the Kaibab limestone located about a quarter mile up the rim trail from the beginning of Hermit Road. The ranger provided a key to the fossil shapes and let the kids search for the different types of life fossilized in this bed.
Learning about the two billion year history behind the formation of the Grand Canyon caused me to contemplate on the timescales of the planet, animal life and human life in a way I never stopped to consider before. As deeply upsetting as I find the destruction humans are wreaking on the natural world, the existence of the plants and animals we share the planet with is just a blip on the geological timescale, the timescale of the planet. If we are indeed driving a mass extinction event, as it appears that we are, it will be the sixth known mass extinction in the history of the planet. Some species will survive and the biodiversity of life will regenerate.
Pictures of each of the rock samples in the Trail of Time follow, from the oldest to the newest.