Originally published 10 December 2002
I spent my childhood clapping erasers in the playground after school.
Funny enough, it was not as punishment for misbehavior. At my school, the teachers had the unusual notion that clapping erasers was a privilege to be awarded to teacher’s pet. And so I sat in clouds of chalk dust, making patterns of white rectangles on the blacktop pavement or the red brick walls of the school.
As a teacher now myself, I’ve spent most of my life covered in chalk dust. It’s beyond me how anyone can teach without a broad expanse of chalkboard and a box of multicolored chalks. Especially in the physical sciences, a good chalk diagram can convey more information than a thousand words delivered from a lectern.
No science benefits more from colored chalk than geology.
In 1815, a self-taught geologist named William Smith published the first geological map of a sizable part of the Earth’s surface, the island of Britain. The complete map was 8 feet by 6 feet, published in 16 separate sheets. Four hundred copies of the map were printed, and Smith colored many of them by hand himself.
The map is a multicolored thing of beauty, and its making is described in Simon Winchester’s recent book, The Map That Changed the World. The story is a good one: How the impoverished son of a blacksmith discovered more about the rocks of Britain than a passel of university-educated gentlemen geologists.
A reduced-size poster of Smith’s map is available from the Liverpool Geological Society. Placed side by side with a modern geological map of Britain, one sees how much Smith got right; the geology is almost identical. The modern map also follows the color scheme invented by Smith for designating different kinds of rocks.
One of my favorite things to do when I used to teach Earth science was to take my students on an imaginary walk across southern England. I drew a multicolored cross-section across 20 feet of chalkboard, showing the ups and downs of the landscape, the sights we might see along the way, and — using Smith’s map as a guide — the rocks we would find exposed at the surface of the Earth.
Then I asked the students to guess at the three-dimensional arrangement of strata that would explain what we saw on the surface. Now the colored chalk came out in force, as we drew sweeping folds of rock — mostly sandstone and mudstone — rising and falling across the chalkboard. Some of the hypothesized folds, of course, have long since been eroded away.
We ended our imaginary walk where William Smith ended his remarkable mapping achievement, with a grand view of the changes that have taken place on the surface of the Earth over eons of time.
As a finale to this exercise, I read the passage from Charles Darwin’s Origin of Species, in which he describes standing on an escarpment near his home in southern England looking out over a broad valley created by the erosion — grain by grain, year by year — of thousands of feet of rock.
“During each of these years,” Darwin wrote, “over the whole world, the land and the water has been peopled by a host of living forms. What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years!”
I had occasion recently to share this imaginary walk through southern England with a group of Earth science teachers. I had brought my own box of many-colored chalk — those pale watercolory hues so familiar to geologists since Smith — to the middle school where we gathered, but found to my dismay no chalkboard.
Instead, as I should have anticipated, the classroom designated for our meeting was equipped with a small whiteboard only, and three or four dry erase markers in primary colors. Fortunately, a teacher from the school led us off to a room in the basement with what may have been — for all I know — the last real chalkboard in America.
We had a grand time, walking the walk, choking in chalk dust.
I guess I retired from teaching just in time. There’s no way I could get used to dry erase markers. The colors are not subtle, and you can’t turn a marker on its side to fill in broad expanses of color. And — well, there’s just something about teaching science in a cloud of particulate matter, a good dusty reminder of the materiality of the universe we are describing.
In 1868, Darwin’s champion, Thomas Huxley, gave a lecture to the workingmen of Norwich, England, titled On a Piece of Chalk, that has come down to us as a masterpiece of scientific exposition. Norwich sits on a chalky stratum, and from a piece of the local geology Huxley teased out a tale of the geological and biological evolution of the Earth over the “long roll of years.”
One wonders what sort of story he might have made with a dry erase marker?
