March 30: The Great Unconformity

A birder friend, Bryan Pfeiffer, recently hiked for several days by himself in the Grand Canyon. (You can see a beautiful one-minute slide show of his adventure at his blog, The Daily Wing.) Although I have visited the South Rim of the Grand Canyon, I wasn't fortunate enough to hike more than 1/2 mile down into the canyon, not far enough on the trail we followed to reach The Great Unconformity. A photo in Bryan's slideshow brought back my long-held fascination with this geological landmark that I first learned about in college geology classes and still hope to see someday in person.

Some background: An unconformity is the division between a younger rock layer and a much older one--a gap in time representing loss, because what it means is that there is no geologic record of the period between the two layers. For whatever reason no sediment was deposited during this time, or what was deposited was somehow washed or blown away. The cool thing is that when you put your hand on an unconformity, you're spanning millions of years. The Great Unconformity in the Grand Canyon represents a missing gap in Earth's history of 250 million to 1.2 billion years--an unfathomable chunk of this planet's history. It's mind-blowing, really. Above the Great Unconformity is sandstone with fossils of shells; below is Vishnu Schist, which formed in a time when the only creatures around to leave a fossil record were bacteria. 

What I didn't remember from those geology classes so long ago (though just a blink of an eye in geologic time) is that the Great Unconformity exists around the world. The Grand Canyon just offers some of the best exposures of it. 

Rock above, below;

one hand spans a billion years.

We are all mere dust.