A walk in Deep Time
A couple of weeks ago a colleague and I, along with twenty-eight students, walked two billion years back into the past. Not literally, of course. What we actually did was to walk the Trail of Time interpretive trail along the South Rim of the Grand Canyon, near the Yavapai Geology Museum. Open in 2011, the trail starts with the “Million Year Trail,” where initially each meter walked is one year in the past, then ten years, eventually increasing up one-hundred-thousand year intervals until reaching one million years ago. At that point, every meter, marked by a brass marker in the ground, takes you a million years back in time. At corresponding intervals are exhibits of rocks of that age from the canyon (the stromatolites, dome shaped structures formed by single-celled organisms, are spectacular), as well as viewing tubes that point to their locations in walls of canyon. The trail ends a virtual two billion years in the past, some 2 kilometers from its beginning. For our students, it was a great introduction to the geological sequence in the Grand Canyon, including its huge span of time and the many gaps that punctuate the rock record there. It was also good preparation for our hike down into the canyon the next day.
The Trail of Time is a very impressive exhibit; the people who conceived, developed, and installed it deserve kudos. It is also a praiseworthy effort to confront one of the biggest challenges faced by geologists when discussing our science: how do we explain the truly vast amounts of time encompassed by the history of the Earth? How long, really, is a million years? A billion years? (I will discuss the related question of “how do we know?” in a future essay). To quote Charles Darwin, writing even before the true scale of geological time was known, ““… it is highly important for us to gain some notion, however imperfect, of the lapse of years.”
This lapse of years, this immensity of time, was dubbed “deep time” in 1981 by John McPhee. As he put it, “Numbers do not seem to work well with regard to deep time. Any number above a couple of thousand years — fifty thousand, fifty million — will with nearly equal effect awe the imagination.” Or to quote Douglas Adams, referring to the equally awe inspiring concept of deep space, “Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the drug store, but that’s just peanuts to space.” To restate Adams, you may think it is a very long time to wait for the bus, but that is just peanuts to deep time.
Two fundamental approaches have been used to convey the immensity of deep time. The first of these is to equate time to distance. For example, Steve Gould, paraphrasing McPhee, “Consider the earth’s history as the old measure of the English yard, the distance from the king’s nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history.” Yet another metaphor, suggested by the Evolution and the Nature of Science Institutes, imagines shrinking ten years into a millimeter. A million years would then be the length of a football field and 500 million years would be thirty miles away. Similar approaches have been used to convey the size of the solar system; a one-to-ten-billion scale model was installed on the National Mall in 2001.
The other approach is to rescale geologic time to some measure of human time. For example, various efforts have been made to portray geological time as a one, twelve, or twenty-four hour clock; e.g., if geologic time started at midnight, our species appeared at one minute before noon. You can also find graphics rescaling the geologic time scale relative to human lifetimes or to one year.
I do not know how effective either alternative approach alone is at really conveying the true length of geologic time. I suspect that they succeed best in communicating the relative lengths of time and sequence of the divisions of the time scale. What makes the Trail of Time so effective is that it combines both the metaphor of distance (how far you walk) and the rescaling of time (how long it takes to walk). It is by far the best approach I have encountered to convey an intuitive “notion, however imperfect, of the lapse of years.”
For more on the Trial of Time, see:
Karlstrom, K., Semken, S., Crossey, L., Perry, D., Gyllenhaal, E. D., Dodick, J., Williams, M., Hellmich-Bryan, J., Crow, R., Watts, N. B., and Ault, C., 2008, Informal Geoscience Education on a Grand Scale: The Trail of Time Exhibition at Grand Canyon: Journal of Geoscience Education, v. 56, no. 4, p. 354–361.
