Complex sentence: Of the 25 crew members, 19 made it ashore, but only three survived the winter, the others succumbing to exposure; they were unaware of the presence of the "Grafton" crew castaways to the south.
Split into simpler sentences: Of the 25 crew members, 19 made it ashore, but only three survived the winter. The others succumbed to exposure; they were unaware of the presence of the "Grafton" crew castaways to the south.