Perceiving Geologic Time

I’ve been teaching at the University for fifty years now and have grown accustomed to folks having difficulty with geologic time. The notion of toothy pterodactyls soaring overhead and T Rex thundering about seems more the stuff of mythology, more in the category of Zeus hurling down his lightning bolts from the clouds, or a sky bear biting chunks out of the moon. But when we manage land for conservation purposes, we are trying to perpetuate systems and processes that have been in place for thousands and millions of years, which then support the plants and animals we list as priorities. So today, let us take a little stroll back in deeper time.

The growth and destruction of mountain ranges is a balance between the rates of uplift and the rates of erosion, over great spans of time. In the Himalayas, uplift still dominates, in the Appalachians erosion has the upper hand, and the Rocky Mountains are a mix somewhere in between. The layered bedrock of our area has its origins from rivers draining westward from the Appalachians hundreds of millions of years ago, washing sediment and dissolved minerals into the shallow sea that was eastern Iowa.

If you visit the mountains of Colorado and kick a stone off a ledge and watch it go rattling down to the river below, you have become a geologic agent; doing what gravity and its assistants were going to do anyway. Your stone joins billions of others, big and small, in the valley bottom, waiting for the next big flood to transform them a bit more.

In 1976, Dick Baker and I drove with a class up Big Thompson Canyon, in Colorado, on the day the road finally reopened, three months after a flood had raged through the canyon. It was clear that boulders the size of small cars had been moving briskly because some were still sitting in the remains of house foundations. The smaller gravel along the banks of the little river contained spark plugs and door handles from automobiles, which had been pulverized by the big tumbling rocks. A 100-year flood may seem like a rare event to us, but if you are willing to view it over 100 million years, it will happen about a million times, quite adequate to really grind down big ones into little ones.

The bed of the Platte River, in its headwaters in the mountains, is full of big rocks. But as you track it eastward, the average grain size diminishes to cobbles, then pebbles, then granules, then very coarse sand, and by the time you get to Omaha it is medium-fine sand. And if you recall my story about bird grit, little birds need sand grit, big birds need larger sizes. One of my classes spent a couple of days camping beside the Platt River near Kearney, Nebraska, to study this river, which is “a mile wide and a foot deep.” And one of our conclusions was that the half million cranes that pour through here each March have picked this spot in part because the Rocky Mountains have been finally crushed and worn small enough to be useful to them as grit, but not too small.

So some long-term geologic processes, such as sediment load evolving down the Platt River, are easy to envision. Others remain elusive to a person just relying on their field observations. For example, during the past fifty years, plate tectonics has carried Iowa City at least ten feet further from London, and I’ve never even noticed.

While standing in a swamp listening to cranes overhead, I start imagining myself in a time machine with the dial set on Cretaceous. Gary Larson, in his Far Side world (pg. 18), does also. Try it.