By Graham Cogley
I was wondering about when the first measurement was made on a glacier. This is probably a diffuse thing to wonder about, because you can measure properties of the atmosphere and even of the solid Earth as a whole while standing on a glacier. You could, for example, measure the air temperature or the air pressure, the latter giving you a decent chance of estimating the surface altitude.
So I sharpened my focus slightly, to the first measurement of a glacier, but still on the glacier. A measurement of retreat or advance of the terminus might be very interesting, but it would not count because it would be made from in front of the terminus. But a measurement of the glacier’s velocity would count.
The idea is fundamentally simple, and is conveyed well by one of the terms we use for it: feature tracking. Identify some feature on the glacier surface that is easy to see, such as a boulder, a particular crevasse, whatever. Measure its position accurately once, relative to an immobile reference point or baseline on land rather than on the ice. Measure it again at some later time. The distance between the two positions divided by the time between the measurements is the glacier speed. The velocity (that is, the speed and the direction, both together) requires only the simple trigonometry that you needed anyway to work out the distance.
The only problem might be lack of features that are easy to see. So make your own feature. As long as it is immobile relative to the ice, and you have a fixed point from which to observe it, any artificial object will do. For 240 years the object of choice has been a stake, jammed into the glacier by brute force or, much better, lowered into a hole drilled for the purpose.
Simple as it is, the idea had yet to occur to anyone at the time — the late 13th century — of the description in Marco Polo’s Travels of the glacier on Mount Ararat.
Apparently the idea that the motion of glaciers presents a problem did not arise until the 18th century. One intellectual roadblock was the need to get clear on the two motions in question. First, the glacier can get longer or shorter. That is, its terminus can move forward or backward, but this is an intellectual trap. The motion of the terminus depends on the mass balance. If more ice arrives than melts or falls off, the terminus advances; if less ice arrives, the terminus retreats.
I smuggled the second kind of motion into that description of the first: the ice can only “arrive” if it is itself in motion. The velocity of the ice and velocity of the terminus are two different things.
Once in a while I come across someone who is surprised to learn that No, the ice itself does not go backwards up the valley. The ice is obeying the forces that are driving it — gravity, pressure and the frictional resistance of its bed. There is no force that can make it flow backwards. But I am not surprised at this surprise in someone who has never had to think about the problem before. It is a tough nut, and it seems to have taken some decades to crack it.
The first person to assert that glacier ice moves was Peter Martel, writing in 1742. His assertion did not go unchallenged. At least one critic thought that ice flow was impossible. On the other hand, at least one interested person thought that a good approach to the question was to look into it. In November 1772, at the instigation of Pierre-Michel Hennin, three stakes were placed in the Mer de Glace on the northeast flank of Mont Blanc. The next spring, it seems, they had advanced about 4.5 metres with respect to a fir tree on the valley side.
So that is our first serious record of a measurement of a glacier. It is a bit of a pity that it was probably flawed. In 1842 James Forbes, one of the giants of 19th-century glaciology, measured velocities more than ten times as great at nearly the same place.
But that Hennin got it wrong isn’t really the point. The point, grasped by Hennin and made repeatedly, and forcibly, by Forbes, is that if you want the truth about a matter of fact, the best bet is a measurement.