Complex sentence: The flints in flushwork areas are very carefully knapped (trimmed to shape by chipping) and selected for fit and consistency of colour, often forming a notable contrast in these respects to those in nearby plain wall areas.
Split into simpler sentences: The flints in flushwork areas are very carefully knapped (trimmed to shape by chipping) and selected for fit and consistency of colour. The decorative patterns often form a notable contrast in these respects to those in nearby plain wall areas.