AS OBSERVED IN THE WEDDELL SEA. 
813 
ridges certainly were less prominent in a few days, and soon lost their rawness and 
angularity by getting drifted over with snow. 
Formation of Cracks by Pressure. — Two kinds were definitely established : — 
(a) Weight or hinge cracks. 
( b ) Shock or concussion cracks. 
Possibly there was a third, namely 
(c) Torsion cracks. 
(a) Ice of advanced age was no longer plastic and did not bend, therefore, under 
the weight of a heavy pressure ridge piled on top of it, as it might once have done. 
The result, if the breaking strain was reached, was a longitudinal crack in front of 
the pressure, the ice being pressed down in much the same way that a door opens on 
its hinge (fig. 4). At the same time a number of radial cracks were developed ; and 
the ice in front of the pressure was thus broken up beforehand into blocks of suit- 
able size for further hummocking. Between a pressure ridge and a weight crack of 
Hinqe Crack 
/_ 
. ~ Water Level. 
Fig. 4. 
this nature the surface of the ice was often below the level of the sea ; the pool 
formed, however, was apparently concentrated brine rather than sea-water. 
(b) The shock or concussion crack, -like the smaller radial cracks mentioned above, 
was also transverse to the advancing pressure ridge. It is sufficient to state that 
cracks of this nature were seen to form on many occasions (p. 811), and that the 
proximate cause was the impinging of one moving floe on to another relatively inert. 
Probably the development of the crack was made easier by the passive floe being 
already in a state of tension. 
(c) All the above types of crack, whether due to outside causes {i.e. pressure) or 
to causes in the floe itself ( e.g . stresses), were actually observed and are beyond all 
question. Torsion cracks are, however, more hypothetical. They have been discussed 
by Arctowski and Hobbs in an attempt to explain leads taking the form in ground- 
plan of a chain of pools. Citing Datjbree’s well-known experiment on the production 
of two sets of cracks at or nearly at right angles, Arctowski makes the claim that 
the same thing may occur in an ice-mass relatively passive. On opening, a zigzag 
lead would result (fig. 5) ; and as it opens farther there would in the ideal case be a 
chain of diamond-shaped areas ; but, as the effect in nature can hardly be as perfect 
as this, it resolves itself (so it is claimed) into a chain of pools. On one occasion a 
sketch was made of some opening cracks in the neighbourhood of the Endurance 
which seemed to be a case in point (fig. 6). The whole matter, however, requires 
