Complex sentence: The French Prealps ("Préalpes") are the group of mountain ranges of medium elevation at the periphery of the French Alps, stretching from Lake Geneva south-west to the Drôme River.
Split into simpler sentences: The French Prealps ("Préalpes") are a group of mountain ranges of medium elevation. They are located west of the French Alps, stretching from Lake Geneva south-west to the Drôme River.