Fox Glacier Marginal Moraine
Page 1 of the giant panorama

This is the edge of Fox Glacier, down near its terminus. The edge is made of rocks jumbled together. The photographer has climbed to the top of this rock heap and could not easily go further because in the centre of the picture, ice reaches the steep valley wall and he had no crampons or ice axe. Only madmen walk on a glacier this size without proper equipment.

The ice is not easily recognised here at the glacial edge, for it is covered by rock and soil debris which has fallen from the valley walls. These rocks have any  angular shape, since they have not been smoothed by water or faceted by ice, they are just the result of rock falls.

These rocks are schist, formed by heat and compression of sea-floor sediments, which sank into the earth's crust, only to be thrust up here by the alpine fault. The characteristic feature of schist is stripes. Schist makes pretty houses and good rock walls, but is not wonderful for mountain building (or rock climbing) as it is not strong enough.

The glacier does not confine melting and break up to its terminus, far below here. It gradually gets thinner and narrower until it leaves the valley wall, dropping the load of rocks as it melts. These rocks form the marginal moraine or kame terrace and become the bed of a marginal stream running beside the glacier when it rains. You cannot make out the terrace here, but further down the valley we will see lots of terracing.

Here is a birds-eye view of Franz Josef glacier showing how it tapers to its terminus, leaving a margin of broken rock.

Photographer Thomas Lowell:

http://tvl1.geo.uc.edu/ice/Glacier.htm

The original web site called this Fox Glacier, but Thomas Riker kindly pointed out to me by e-mail that this is really Franz Josef Glacier. Indeed it is: the river from Franz Josef flows on the true right side of the valley, as here, but the Fox river flows to the left. There are other differences in the surroundings, not so easy to see because of the dark exposure used for this picture (which brings out ice detail nicely).

 It is a mistake to think glaciers are slowly moving rivers of ice, for ice is only part of it. A major component is rocks which drop onto the ice from the steep walls of the valley, or which the glacier drags out of its bed.

Ice cubes are slippery and slide around on rocks with no damage to the stone, but some melting of the ice. Obviously this is not the mechanism for:
glacial erosion of mountains.

  1. The ice may freeze to the rock bed.
  2.  Eventually, when it is thick and heavy enough, the ice moves down hill under the influence of gravity, dragging stones of all sizes out of its frozen bed.
  3. The stones are held by the ice and act as engraving tools to cut the rock over which the glacier is sliding.
  4. Since stones do the work, the bed-rock is engraved with grooves, which show the direction the ice was moving.
  5. Stone fragments of all sizes in the bottom of the glacier contribute, so the bed-rock becomes smooth and gently rounded as if rubbed with sand-paper.
  6. The "engraving tools" themselves get ground down and develop facets (flat surfaces) to become "flat-iron rocks".


 Faceted rock from the base of a glacier.
 After Cotton, 1942.


 Faceting even extends to the fine sand grains, which have flat faces and sharp edges under a microscope. Quite different to river pebbles and sand, which roll and rub to become smoothly rounded.

The grinding on the glacier's bed is so severe, much of the rock is reduced to "rock flour". New Zealand glacial rivers and lakes are milky from suspended "rock flour" which takes so long to settle that even large lakes stay milky (Notably, Lake Pukaki, fed by the huge glaciers of Mt Cook National Park). When the lake and sea beds are finally exposed up to centuries later, the rock flour is so fine it is blown by  the wind as loess. The Canterbury plains are made of outflow gravel from glaciers and rivers, covered by loess. 

Already recently ice-covered rocks beside the glacier are collecting life forms, note the golden colour of lichen, low center.

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