Complex sentence: But subsequently the river's course migrated south, away from Grange, allowing nature to develop the "sands" (mudflats, in truth, with dangerous quicksands at uncertain points) into a grass meadow now frequently grazed over by small flocks of sheep.
Split into simpler sentences: But subsequently the river's course migrated south, away from Grange. The "sands" (mudflats, in truth, with dangerous quicksands at uncertain points) became a grass meadow now frequently grazed over by small flocks of sheep.