Serac ice blocks. Sunlit clouds over ice blocks on the Argentiere Glacier in the French Alps. The movement of the glacier has caused the upper surface to break up into large blocks called seracs. This glacier flows some nine kilometres from its source towards the valley of Chamonix valley, terminating near the village of Argentiere. Glaciers can be sensitive to climate change and this glacier is among those that in which rapid retreat (loss of ice) has been observed.