Complex sentence: Possibly the apparent "mountains" of "Crocker Land" may have have been caused by icebergs that were frozen into the pack ice, or were caused simply by the uneven surface of the ice itself.
Split into simpler sentences: Possibly the apparent "mountains" of "Crocker Land" may have have been caused by icebergs that were frozen into the pack ice. On the other hand they may simply have been caused by the uneven surface of the ice itself.