As I say, I’m not a good record-keeper.
So I marvel at people who can keep highly ordered tax papers, favorite family recipes, instructions on how to run their appliances, notes on when they last had the furnace serviced, etc.
But in some ways Mother Nature keeps better records than we ever shall, at least when she wishes to. And she keeps them, potentially, for billions – literally billions – of years.
I’ll explain this feat by way of my favorite seasonal analogy: food.
Consider the last time you bellied up to a long and varied brunch buffet. I recently did this at church. (I may miss midnight services, but never brunch buffets.) You may have done it at a commercial establishment of the all-you-can-eat variety. In any such place, if you are lucky, you had an egg-and-bread based entry called strata.
Besides eggs and cheese, strata ingredients may be ham, Canadian bacon, a few select veggies like spinach, onions and tomatoes, or (if you are particularly lucky), all of the above.
Strata gets its name from the layers of bread in it. It’s like a layer cake, except hot and full of protein rather than sugar. What could be better, in particular as late autumn slips away toward the first day of winter?
To a geologist, each bread layer in the dish is a stratum (singular). The more strata (plural), the better. Naturally, you place the lowest stratum into the pan first. Next the middle one, finally the top one.
When Mother Nature makes sedimentary rock – the greatest of all record books on Earth – she “lays down” a single stratum first. It’s likely to be a layer of sandstone (made of sand grains) or shale (made of mud) or limestone (which may be made almost entirely of shells) or coal (made of ancient plant remains). The next stratum, of necessity, goes on top the earlier one. So the second stratum is younger than the first. Similarly, a third stratum above is younger than the first two, etc.
If you can conjure up an image of the Grand Canyon in your mind’s eye, you know that there are places on Earth where dozens of layers are piled up, one atop the next. Those places are heaven to geologists. They are strata-Meccas where we go to get inspiration – and to retire when we are just too old for any work beyond baking and eating strata (the egg dish, I mean).
It’s the basic ordering of the sedimentary layers – oldest, middle, youngest – that gave geologists our first way of fully unraveling the history of the Earth. Essentially, we dated (and named) all the sandstone and mud layers, listing them out in terms of which happened first, second, third, fourth, fifth, sixth, seventh and so on, all around the whole Earth. (Yes, we did the work patiently and carefully, and it really worked. Some geologists can keep much more careful records of their work than others!)
It’s because the strata of sedimentary rocks contain fossils that we developed not just a history of mud and sand, but of swimming and crawling and even flying creatures, as well. That’s how we “got the picture” that great fishes came first in the fossil record, followed by amphibians and reptiles, then followed by dinosaurs and mammals and birds.
And that’s what impresses me so much about Earth’s strata. They taught us the grand story of life. The record of life, sometimes including delicate fossils, was maintained over epochs, eras and eons in those layers. Although the record of fossils and sedimentary rocks have to be interpreted carefully, no paper records or programming on DVDs could ever compare in full and rich complexity to what Mother Nature preserved – and that we can study whenever we wish.
Dr. E. Kirsten Peters is a native of the rural Northwest, but was trained as a geologist at Princeton and Harvard. A library of past Rock Doc columns is available at