AS OBSERVED IN THE WEDDELL SEA. 
807 
Weyprecht gives a good instance to show how a patchwork field of pack-ice 
is always in a state of strain. He postulates the case of a fioe 2 metres thick 
being divided by a crack, where young - ice immediately starts forming. It 
will soon be 1 metre thick, and by then the proportion above water should be 
0‘2 metre (taking 4:1 as the average proportion of submerged ice) ; but the 
thicker, earlier-formed ice bordering the crack has only increased perhaps one- 
tenth of this amount, and consequently in its case only a further 0'02 metre 
should emerge. So at the junction either the younger ice is held down below 
its proper level., or the older ice is buoyed up. In any case, there is a stress of 
some sort along the line of contact. 
Sea-ice when newly formed is extremely plastic and can conform to such stresses 
as these ; but as it ages it becomes much harder and less bendable, so that finally 
it must reach that state of strain at which it only requires some slight impulse 
to break it. In most cases this impulse will be the wind, though occasions must 
occur when the ice cracks simply because it has passed the breaking strain. 
Proof of tension in a floe before it cracked was forthcoming in many ways : the 
ice-surface on one side of a freshly formed crack might immediately take a different 
level from that on the other side; the two sides might be displaced laterally; or, 
again, the opposite sides of a crack might open out at once to a breadth of some 
inches, to become stationary at that distance for a considerable time. 
A special case of strain cracks, and one easily realised, occurs when a swell 
from the open sea runs under a wide ice-field. Over the trough of the wave* 
a thick ice-floe must be entirely unsupported ; a crack parallel to the wave-front 
may result ; when more than one such crack occurs, they will of course be 
parallel to each other. A case in point occurred on March 30, 1916. Whether 
when more than two in number they will be at equal distances apart is open to 
doubt ; should it be so, the interval from crack to crack may possibly be half 
the wave-length. During the Endurance drift, never more than a couple of 
parallel cracks, originated beyond all doubt by a swell, were seen. This was 
probably due to the fact that, by the time the floes drifted into the region of 
swells from the open sea, they were already much reduced in size by the 
formation of ordinary strain cracks. 
There were cases during the winter, however, when series of cracks formed 
whose relationship was so nearly parallel as to make one inclined to invoke the 
agency of a swell running under the pack from open water. The best of these cases 
was a series of four cracks (one of which ultimately developed into a lead) which 
opened on March 17, 1915 (fig. 3). Seven miles to the south-east there was open 
water on Maxell 11, and beyond that open water as far as could be seen, perhaps 
even as far as Coats Land, thirty miles farther on. A swell was therefore well 
within the bounds of possibility. The cracks were not parallel, however, but seemed 
to form a fan-shaped group. This suggests another explanation, namely torsion. 
