Jökull


Jökull - 01.12.1953, Síða 32

Jökull - 01.12.1953, Síða 32
Four stagesinthe formation ot an end moraine Accumulation of dirt on snout Recession due to melting ** Isolated ice blocks leftunderdcbris. Snout recedes Fig. 6. The formation of an end moraine at a receding glacier snout. Jökulgarður myndast við jökultotu, sem er að bráðna. This is not in accordance with Carruthers (1943). The lack of much englacial material is contrary to the impression of Sharp (1948). MORAINES. Medial Moraines examined by the expedition are described in another section of this expedi- tion report. Moraines were traced to the source and development noted as far as possible. Ice was sampled for dirt content in, near and below different moraines. The moraines were princi- pally a surface feature which is contrary to pres- ent concepts (de Martonne and Flint), and had little form until the surface cover of grit was sufficiently thick to protect the ice under it. Mr. D. E. St. A. Harney made the only real contribution at this stage in that he found a vertical dirt band beneath a medial moraine and suggested that the few cms. wide dirt band was the boundary between two glaciers and the dirt was collected when the glaciers were each in contact with a valley side or nunataks. The melting down of the surface permits the dirt to spread out and become a moraine. End Moraines were of all sizes some of which had wet, steep sides on the hottest days, the water coming from the dead ice within the moraine. Much morainic deposit had been re- sorted by the glacial streams and rivers which braided and changed their beds with amazing frequency. Along the glacier snout the angle of steepness of the ice varied considerably. It was only at the thinner, less steeply sloping parts that there were definite linear deposits close to the ice edge. Long wedge shaped slabs of ice were deposited a few metres from the glacier snout along a stretch of its northern margin. The ice observed for bottom-melting appeared to have been recently broken off the glacier snout. The underside of the glacier was vaulted as far as one could creep in to see. Each point between the arcs seemed to be supported by the rocks under the glacier but fluvial action sometimes removed these supports and large pieces of ice slumped to break off from the main body of the glacier snout. A suggested sequence of events in the förma- tion of a terminal moraine is outlined in Fig. 6: (I) Rock debris of all sizes, particularly the coarse grit, accumulates on the lower slopes of the snout. The main drainage being in channels and crevasses leaves most of the debris between these channels relatively undisturbed. (II) The meltwater sorts the debris at the ice edge and under the ice and accelerates undermelting of the snout edge. The water under the snout is added to from tension cracks, moulins, etc. The debris on the ice edge is relatively undisturbed between the gaps where the meltwater emerges. (III) The decrease in surface melt leaves under- melt more dominant which causes cont- inued thinning. Removal of the supports under the ice edge by water action or by forward movement of the glacier (particu- larly during winter) causes parts of the ice edge to fracture due to shear of the canti- lever. (IV) The ice blocks which are broken off sink into the waterloggod debris. Some debris may be added to the top from off the snout until the snout recedes due to melting. CONCLUSION. The results indicate that much more quantita- tive data is required on ablation during periods of good weather and on quantity and forrn of rock debris present in moraines. The apparatus used has been modified in the light of experi- ence gained yet still keeping expense within 30

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Jökull

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