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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
size, retain sufficient strength to exert a strong mechanical pressure on the 
shoreline and on the bottom of the bay near the shore. 
A case has been observed when such ice destroyed, in a brief period, a jetty 
of solid construction. During the work undertaken to thaw out the mooring 
we had a chance to convince ourselves of the strength of black-ice blocks, which 
did not crumble even under the caterpillars of an eighty-ton tractor. 
Southward migration of black-ice along the western shore of the Ob inlet, 
which melts away on the shore or in its vicinity, provides a good reason to 
believe that such ice carries a considerable portion of the overburden settling 
in the bay. At the same time, observations of the action of black-ice on the 
shores lead us to the conclusion that this action is one of the factors determining 
the specific features of the shore outline in the Ob inlet. These peculiarities can 
be clearly seen on the planimetric aerial photographs of the western shore of 
the inlet; all large and small rivers discharge into the inlet in a direction 
opposite to that in which the Ob water flows; all the sand spits in the mouths of 
the rivers are elongated from north to south, whereas it would be more natural 
for them to build up in an opposite direction. 
Without belittling in any way the importance of wave activity, which also 
has a definite effect on the shaping of the shoreline, and considering the 
effect of the water being driven in and away by strong winds, we cannot at the 
same time ignore the facts revealed by the aerial photographic survey. These 
facts irrefutably show that the ice of the Ob inlet plays a certain part both in 
the shaping of the shoreline and the formation of coastal shoals. The sketch- 
maps in fig. 7 enabling a comparison between the zones of disappearing 
black-ice and the distribution of the coastal shoals, fully corroborate this assump 
tion. It should also be mentioned that in a volumetric sense, extremely large 
amounts of overburden settle annually near the western shore of the Ob inlet 
after the black-ice has disappeared. According to computations, the accumu 
lation of overburden reaches a volume of 25,000 cubic metres in an area of 
one sq.km, with a sand layer of 25 mm thickness. 
In a number of cases the break-up of fast-ice does not occur spontaneously 
nor in arbitrary directions but, as the aerial photo interpretation has shown, it 
is controlled by wave movements of the water masses. The interference of 
direct and reflected waves causes thread-like cracks to appear in the fast-ice. In 
spring, the thickest elongated névés develop along these lines of cracks and 
form a combination of parallel dash-like lines on the ice surface (fig. 8). 
The skeleton diagrams of the distribution of these lines obtained from 
vertical aerial photographs (fig. 9) do not distract the attention of the observer 
by abundant particulars of secondary importance peculiar to a photographic 
image and help us to understand the nature and origin of these thread-like 
cracks. 
The lines of cracks are not exactly straight. They are distinguished by a double 
magnitude curvature whose mean radius varies between 7.8 and 9.2 km. 
Although these lines are not quite uniformly distributed over the ice surface,