The Meaning of a Rock Wall

I have just returned from spending about a week hiking around and looking at one of the most spectacular Mesozoic formations on the earth [1]. In some places the stratigraphic column is hundreds of feet thick with clear boundaries between the Triassic, Jurassic and Cretaceous [2]. The Triassic red beds, the Jurassic sandstones and the mix of Cretaceous rock types make high canyon walls and cliffs running for many miles. They are mute rock record of over 150 million years of the earth’s history in this region. The lowest and oldest rocks in the column, Triassic, have yielded up thousands of dinosaur, reptile and amphibian fossils; as a fossil hunter, walking in the red beds is thrilling, but it was the Jurassic sandstones that, this time, took my attention and imagination.

From one end to the other of this exposure the lower peach-tan layers shade into a wheat color and can be more than 100 feet thick. That 100 feet is broken into bands with different patterns of deposition: many feet of solid sandstone and then a band of cross-bedded deposits with a truncated top surface and more cross-bedding, another layer of solid, undifferentiated deposition and then more cross-bedding.

Sitting in a narrow canyon, high up on one wall looking directly at the opposite side straight on, I tried to imagine the processes that laid down these layers. A geologist with us said that there were disagreements about whether some of the sediments were marine, littoral zone or dune in origin, but most were considered to be dune deposits adjacent to lagoonal seas.

As I pondered this, looking at the massive wall of cryptic history in front of me, I was struck by the incongruity of the actual actions of water, wind and sand and the 50 plus million years of the Jurassic period. The wall was huge, but the Jurassic was long. I calculated that if one grain of sand thickness (one millimeter) were deposited every year for the whole of the time, the total distance would be 50 kilometers [3]. Yet, there was only 100 feet of rock for the whole of the Jurassic. I could see in the fine layers of the cross-bedded dune sands that each tiny band of sand, followed by hundreds or thousands just like it, must be counting the seasonal cycles of years.

So, there were only thousands of years out of the millions available actually represented in the rock. Millions and millions of years had occurred, been lived through by the earliest birds, mammal-like reptiles, almost modern insects, dinosaurs, the coniferous plants and much more without a single trace, at least here in this place.

Here a thousand years of deposition; there a few millions of years unrecorded; and again a bit of deposition. The land rose and subsided just above and just below the level of the shallow sea. The wind blew and the rains fell. But that was not what was recorded in the rock. The rock recorded the formation of rock and once in a great while it, by the purest of happenstance, captured a raindrop, stored an earthquake, saved the ‘giant bird print’ of a hunting dinosaur or a bone; froze some surface ripples in the sand from one moment in one day and not the millions of other moments in the millions of the other days when sand was washed to ripples by the waves of the shallow sea or by an unrelenting wind.

The beautiful sandstone wall in front of me was not a movie to be viewed from bottom to top. It was millions of disconnected still images, very still indeed, captured in the hard rock. Millions of years of water percolating through the rock and millions of years of water and wind exposed and nibbled at the rock edges, washing out the canyons and river valleys; the form of the images lost irrevocably as the grains of sand separated from the patterns locked together so many years ago.

As I stared across the canyon I realized that the sandstone wall could tell me very little about life on this spot 150 million years ago, but it could tell me something as geological evidence combined with other evidence from other locations, as metaphor and as instrument for measuring my own life.

First me and mine: my 66 year old body still scrambled up the 150 million year old rocks with only minor inconvenience from the old bicycle-injured shoulder. The pleasure in place and beauty was not diminished by multiple exposures; this was and still is my element. The others I was with, many experiencing a bit of rock climbing for the first time, and certainly this scale of earthly gorgeousness for the first time, were properly moved. Grandeur may be a human invention, but it is a powerful one.

The geology is more interesting. The tiny bit of time evidenced for the Mesozoic in this location is supplemented by tiny bits of time evidenced in many thousands of other locations. By combining all of the bits and arguing over how their edges should fit together, geologists have come up with a pretty good, and continually improving, picture of these millions of years of earth history. By knowing the current state of that understanding it is possible to give real meaning to the brief statements contained in this one Jurassic sandstone wall.

But it is the metaphor that moves me most. Thousands of geologists each working within a system guided by a method based on open communication and transparency of process could bring order and understanding to the very most complex problems on the cognitive map. Physics may require deep individual mathematical sophistication and insight, and incredible equipment. Geology requires a vast human infrastructure working with common principles as well as massive powers of abstraction; very human tools.

Just two hundred years ago almost nothing was known about geology beyond basic geography and yet today we have convincing evidence for a 4.6 billion year old earth with a detailed history of slow but constant change [4]. It is a testament to the incredible powers of observation, idea and communication of which our species is capable. And if it is possible to understand something as counter-intuitive as the forces that drive plate tectonics and mountain building, it is possible for groups of humans, organized and committed to effective principles, to comprehend our economics, politics and even ourselves.

We know how to see a sandstone wall: first, realize the considerable complexity hidden in its apparent simplicity; and then bringing our great powers of observation, reason and tenacity together with honest and honorable process – I see no reason not to say it in this way. The women and men who do this work are moved by the grandeur of both the land and the enterprise; this too is important.

Concentrated in this landscape is the power of the earth to, first, overwhelm and then to organize the senses. There is a meaning here too, and it is that we are either participants in the earthly enterprise or we are visitors. Visitors do not need to understand, but also must at some point leave – can overstay their welcome. If we are to stay, we must come to understand.

[1] Mesozoic – middle life. Modern multicellular life is almost 600 million old. The oldest division of that duration is called the Paleozoic, ending about 245 million years ago in a great cataclysm, followed by the Mesozoic, ending in a great cataclysm about 65 million years ago, followed by the Cenozoic within which we presently reside, and a period of time yet to show changes sufficient to name a new period, i.e., there has been no cataclysm worthy of such renaming. It is an irony that the naming agent could be wiped out by such an event and thus no new name attached.

[2] The three periods of the Mesozoic: Triassic (245 mya to 208 mya), Jurassic (208 mya to 145 mya), Cretaceous (145 mya to 65 mya). This was the time during which the dinosaurs became the most representative fossils in the fossil record. The Mesozoic began with the supercontinent of Pangaea and ended with the present continents moving apart toward their present positions.

[3] 50 kilometers (31 miles) would be the thickness with an average deposition rate of 1 mm per year; that is not too different from a year’s worth of dust on an untended windowsill. It may be known exactly how much of the Jurassic is represented in this particular sandstone, I do not, but is considerably less than 50 million years; more like less than one tenth of one percent (.1%) of that time.

[4] I take some license here. While change is always happening, it has in no way been constant; rather episodic: long periods of very gradual movement of earth surface, climate, chemistry and processes punctuated with intense violence from within the earth and from the sky.