Complex sentence: As the ice sheet disintegrated, it created at its front an immense proglacial lake, formed from its meltwaters, as the retreat of glacial margins is not caused by a reversal of the glacier's flow, but rather from melting of the ice sheet.
Split into simpler sentences: As the ice sheet disintegrated, it created at its front an immense proglacial lake, formed from its meltwaters. The retreat of glacial margins is not caused by a reversal of the glacier's flow, but rather from melting of the ice sheet.