Complex sentence: In winter, Bergschrunds are often filled by snow from avalanches from the mountain flank above it, but in later summer due to ablation it lies open and can present a very difficult obstacle to alpinists.
Split into simpler sentences: In winter, Bergschrunds are often filled by snow from avalanches from the mountain above it. In later summer, due to melting, it lies open and can present a very difficult obstacle to alpinists.