By Graham Cogley
You can find some surprising things at the bed of the glacier. Normally it is inaccessible to direct observation, but these days most glaciers are retreating. If you don’t mind waiting a bit — and glacial geomorphologists don’t really have the option — then keeping a close eye on what is emerging can be very informative.
In a paper published recently in Geology, Mark Johnson and co-authors present another surprise: nice fresh drumlins. Múlajökull is an outlet glacier, draining one of the ice caps in Iceland. Like almost every other glacier, it has been retreating. Like only a small proportion of other glaciers, it is a surging glacier — which is going to set the cat among the pigeons when we have had time to think it over and decide whether the surging is relevant. For the retreat of Múlajökull has exposed a field of drumlins.
Johnson and his co-authors were able to show that the drumlins consist of multiple layers of till, sediment carried by the glacier and deposited by a mixture of lodgement — expulsion from the moving ice — and deformation of the sediment over which the ice was flowing. The evidence suggests that each of the till layers represents a surge of the glacier. What is more, at least one of the boundaries between till layers is an erosion surface. That is, the lower layer has been truncated before the upper layer was draped over it.
This is yet another confirmation that the old question about drumlins, “Are they formed by erosion or by deposition?”, was the wrong question to ask. The answer is “Sometimes one and sometimes the other, and often (as at Múlajökull) a bit of both, with some deformation of what was there already mixed in”.
The resemblance of drumlin fields to baskets of eggs has been remarked on before. Lowland Britain is covered with them — tens of thousands of eggs. What is most interesting about the Múlajökull drumlins is that they are new-laid eggs, and the hens are still busy in the coop.
Nobody believes that the drumlins we see today in places like Great Britain and central North America have changed much since the retreating ice margins exposed them to view thousands of years ago. All the same, drumlins that are henhouse-fresh exert a powerful pull on the geomorphological and geological imagination. This is because of actualism, the ingrained principle that the present is the key to understanding the past. The likelihood is that there are lots more drumlins still forming behind the present-day retreating margin of Múlajökull, and as the authors point out we know as yet of no other drumlins that are in process of formation.
One thing that bothers me about the Múlajökull drumlins is that I have trouble seeing the multiple till layers in the photograph that is supposed to illustrate them. But among the reasons why I am not a sedimentologist is that dirt is not very photogenic, and I am prepared to go along with the authors’ interpretation of what they saw in the field. Let us take it that these drumlins are indeed layered, and let us go one step further and accept their evidence that the layers have probably formed during the successive surges of the glacier. (They come along every 15 to 20 years, short-lived advances of a couple of hundred metres, punctuating a retreat that has been going on for about 200 years.)
Does this mean that there is something special about drumlins that are shaped by surging glaciers? Surging glaciers are sufficiently uncommon, and drumlins sufficiently widespread, that it is not likely that surging behaviour is a necessity for drumlinization. It is, however, interesting, and maybe significant, that the deposition probably accompanies the surges and not the longer intervals of retreat, during which there was either erosion or at least non-deposition.
Is there, instead, significance in one or both of two observations made in the Johnson paper: that the drumlins appear to have formed very close to the ice margin, within a kilometre; and that they appear to have formed beneath crevasses that run parallel to the flow direction of the ice? The authors offer only a sketch of an argument for why these associations might be a source of insight. But drumlins have been a puzzle for more than a hundred years. More facts can only help, even if all they do is to make us confused in a deeper and richer way — but especially if they are new-laid facts.