New Laid Drumlins

If I have bone topick here is the assumption that the layering represents something slow acting.  It makes way more sense for a summer’s meltto surge after every rainfall as gathered rainwater fills a channel flushingsediments into the drop zones first fast and then very slowly.  The sediment is constantly replaced by the continuousmelting of the ice.


Thus a drumlin wouldform close to the retreating edge and be heavily layered.  As the ice rotted away the channels wouldopen up eliminating further deposition and limiting erosion.


The layering is actuallyconfirmation of their swift emergence and actual history.


Rain fall gatheringon a glacier escapes down sinkholes and finds its way to surface material andthen burrows its way out forming a river like channel.  It is quite possible that a drumlin is simplythe bottom of such a vertical channel.


New-laid drumlins

You can find some surprising things at the bed of theglacier. Normally it is inaccessible to direct observation, but these days mostglaciers are retreating. If you don’t mind waiting a bit — and glacialgeomorphologists don’t really have the option — then keeping a close eye onwhat is emerging can be very informative.
In a paperpublished recently in Geology, Mark Johnson and co-authors present another surprise: nice freshdrumlins. Múlajökull is an outlet glacier, draining one of the ice caps in Iceland. Like almostevery other glacier, it has been retreating. Like only a small proportion ofother glaciers, it is a surging glacier — which is going to set the cat amongthe pigeons when we have had time to think it over and decide whether thesurging is relevant. For the retreat of Múlajökull has exposed a field ofdrumlins.

Johnson and his co-authors were able to show that thedrumlins consist of multiple layers of till, sediment carried by the glacierand deposited by a mixture of lodgement — expulsion from the moving ice — anddeformation of the sediment over which the ice was flowing. The evidencesuggests that each of the till layers represents a surge of the glacier. Whatis more, at least one of the boundaries between till layers is an erosionsurface. That is, the lower layer has been truncated before the upper layer wasdraped over it.
This is yet another confirmation that the old questionabout drumlins, “Are they formed by erosion or by deposition?”, was the wrongquestion to ask. The answer is “Sometimes one and sometimes the other, andoften (as at Múlajökull) a bit of both, with some deformation of what was therealready mixed in”.
The resemblance ofdrumlin fields to baskets of eggs has been remarked on before. Lowland Britain iscovered with them — tens of thousands of eggs. What is mostinteresting about the Múlajökull drumlins is that they are new-laideggs, and the hens are still busy in the coop.

Nobody believesthat the drumlins we see today in places like Great Britain and central North Americahave changed much since the retreating ice margins exposed them to viewthousands of years ago. All the same, drumlins that are henhouse-fresh exert apowerful pull on the geomorphological and geological imagination. This isbecause of actualism, the ingrained principlethat the present is the key to understanding the past. The likelihood is thatthere are lots more drumlins still forming behind the present-day retreatingmargin of Múlajökull, and as the authors point out we know as yet of no otherdrumlins that are in process of formation.

One thing that bothers me about the Múlajökull drumlins isthat I have trouble seeing the multiple till layers in the photograph that issupposed to illustrate them. But among the reasons why I am not asedimentologist is that dirt is not very photogenic, and I am prepared to goalong with the authors’ interpretation of what they saw in the field. Let ustake it that these drumlins are indeed layered, and let us go one step furtherand accept their evidence that the layers have probably formed during thesuccessive surges of the glacier. (They come along every 15 to 20 years,short-lived advances of a couple of hundred metres, punctuating a retreat thathas been going on for about 200 years.)
Does this mean that there is something special aboutdrumlins that are shaped by surging glaciers? Surging glaciers are sufficientlyuncommon, and drumlins sufficiently widespread, that it is not likely thatsurging behaviour is a necessity for drumlinization. It is, however,interesting, and maybe significant, that the deposition probably accompaniesthe surges and not the longer intervals of retreat, during which there waseither erosion or at least non-deposition.
Is there,instead, significance in one or both of two observations made in the Johnsonpaper: 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 thatrun parallel to the flow direction of the ice? The authors offer only a sketchof an argument for why these associations might be a source of insight. Butdrumlins have been a puzzle for more than a hundred years. More facts can onlyhelp, even if all they do is to make us confused in a deeper and richer way —but especially if they are new-laid facts.

No comments:

Post a Comment