Two days after leaving South Georgia, Endurance encountered polar pack ice and progress slowed to a crawl. For weeks Endurance worked her way through the pack, averaging less than 30 nmi (60 km) per day. By 15 January 1915, Endurance was within 200 nmi (370 km) of her destination, Vahsel Bay. By the following morning, heavy pack ice was sighted and in the afternoon a gale developed. Under these conditions it was soon evident that progress could not be made, and Endurance took shelter under the lee of a large grounded iceberg. During the next two days, Endurance moved back and forth under the sheltering protection of the berg. On 18 January, the gale began to moderate and Endurance set the topsail with the engine at slow. The pack had blown away. Progress was made slowly, until hours later Endurance encountered the pack once more. It was decided to move forward and work through the pack, and at 5:00 pm Endurance entered it. This ice was different from what had been encountered before, and the ship was soon amongst thick but soft brash ice, and became beset. The gale increased in intensity and kept blowing for another six days from a northerly direction towards land. By 24 January, the wind had completely compressed the ice in the Weddell Sea against the land, leaving Endurance icebound as far as the eye could see in every direction. All that could be done was to wait for a southerly gale to start pushing in the other direction, which would decompress and open the ice.
In the early morning of 24 January, a wide crack appeared in the ice 50 yards (46 m) ahead of the ship. Initially 15 ft (4.6 m) across but 1 mi (1.6 km) long, by mid-morning the next day the break was over 0.25 mi (0.40 km) wide, giving the men on the Endurance hope that the ice was breaking up. But the break never reached the ship itself, and despite three hours under full sail and full speed on the engine, the ship did not budge. Over the next days, the crew waited for the southerly gale to release the pressure on the ice, but while the wind backed to the hoped-for south/southwest direction, it remained light and erratic. Unseasonably low temperatures of around −2 °F (−19 °C) additionally kept the ice together. Occasional breaks in the ice were spotted, but none reached the ship and all closed up within a few hours.
On 14 February, an open channel of water opened up 0.25 mi (0.40 km) ahead of the ship and dawn showed the Endurance was afloat in a pool of soft, young ice no more than 2 ft (0.61 m) thick, but the pool was surrounded by solid pack ice of 12–18 ft (3.7–5.5 m) in thickness, blocking the path to the open lead. A day’s continual work by the crew saw them hack a clear channel 150 yd (140 m) long. This work continued through the following day (15 February) and, with steam raised, the Endurance was backed up within her pool as far as possible to allow the ship to ram her way through the channel. As the ship went astern for successive attempts, lines were attached from the bow to loosened blocks of ice, estimated to weigh 20 tons (18 tonnes), in order to clear the path. The pool proved too small for the ship to gain enough momentum to successfully ram her way clear and by the end of the day the ice began to freeze up again. By 3:00 pm, the Endurance had made 200 yd (180 m) of distance through the ice, with 400 yd (370 m) still to go to clear water. Shackleton decided that the consumption of coal and manpower, and the risk of damage to the ship, was too great and called a halt.
Endurance becomes beset, frozen into heavy pack ice a day’s sail from the intended landfall.
Source: Wikipedia