Complex sentence: Cursuses are perhaps the most enigmatic of ancient monuments, they typically comprise two parallel ditches cut to create a "cigar shaped" enclosure, the larger of which can be a mile or more in extent.
Split into simpler sentences: Cursuses are perhaps the most enigmatic of ancient monuments. They typically comprise two parallel ditches cut to create a "cigar shaped" enclosure, the larger of which can be a mile or more in extent.