In March, if there are clear skies and no wind with the sun low even at midday and dipping out of sight at midnight, the sea which is radiating off heat rapidly, drops to a temperature of 28.5 deg F. and begins to freeze. Small crystals of ice form and the water takes on a soupy appearance. These crystals are called Frazil Ice. We used to push through it rather slowly in a steel hulled landing craft called an M Boat, but it is not a good idea to stop as the frazil ice can become a thicker Sludge Ice and in a few hours, freeze solid, and there you are until next spring. Any moderately strong wind will however break it up, some years the sea may freeze many times and break up, often not until May or even June does it become solid enough to sledge on and then only in bays.
As the sea begins to freeze, unless it is VERY calm, it will form platelets growing to plates a foot or two across, with upturned edges where ripples splash and freeze. This is pancake ice.
This is continuously solid and is only a few inches thick and looks green and translucent. One can sledge on it but it bends down a few inches under the weight of you and your sledge so you travel in a moving trough. Another sledge ahead or behind also forms a trough, so you advance on a moving wave. If one holds an ice chisel, which has a handle 6 ft long at knee height and drops it and it does not go through, it is safe. All the same, wear skiis and keep away from the heavy sled! Keep away from the ice edge, from polynya and places where water is showing. Drowning your dogs is a no no!
Do not try to sledge across pack in a strait subject to a strong tide flow for obvious reasons. I belong to the "Antarctic Swimming Club" myself, but the conditions of entry are not pleasant it is like being smacked across the bare belly with the proverbial wet spade.
A wind or tide will drift pack against a shore or against fast-ice, where it will pile up an a tangled line of blocks as much as fifty feet high. One may have to traverse to one side for miles to find a way through. Alternatively spend an hour or two chopping with an ice-axe. A massive pressure ridge of old pack ten feet thick will stop the biggest ice-breaker! One may be able to travel a mile on flat ice with hardly a crack, and then find a huge pressure ridge, or there may be several pressure ridges with only a hundred yards between them. On sea ice you never can tell. One could usually sledge from Scott Base over to Butter Point in two days, but if enough pressures ridges had formed it might take 3-4 days.
In bays sea ice may not break up in the summer if it is sheltered from any swell and may remain for 2-3 years or more becoming a small ice shelf perhaps 10 20 feet thick. Good to unload cargo onto.
By early summer, the pack will be softening and becoming quite rotten losing all its strength. Rotten pack eight feet thick will not carry your weight, you sink through. Rotten ice is especially dangerous to take tractors on, keep your ice chisel handy. Hard ice 8in thick will take a heavy Weasel, rotten-ice ten feet thick will not. Several men have been drowned in McMurdo Sound when taking a vehicle or tractor onto rotten ice or ice that was too thin.
Took us a day and a half to get past the damn thing. We fired a few five inch shells at it, but they just bounced off in a spray of shattered chips ice can take a lot of shock. A little bit only 60 miles long later broke off and drifted to Cape Bird and another bit only a few miles across moved round and blocked off most of McMurdo Sound. Watching it slowly coming closer was like being in a bottle and seeing a cork being pushed in. Four icebreakers the "Glacier", the "Edisto", the "Eastwind", and the "Northwind" tried to push it away. They burned a lot of diesel oil, but I did not see the berg move much.
|__| A large flat-top berg blocking McMurdo Sound.
After a few days the current changed and it drifted off. Icebergs can be detected at night or in fog; hold your hand up and you can feel the cold; as though being radiated off, never mind what the smarties say, you can FEEL it.