Complex sentence: In the Northeast corner of the complex, four circular stone-covered mounds are arranged in a square and apparently functioned as a calendar by using gaps in the walls that are aligned to significant seasonal positions of the rising sun and moon.
Split into simpler sentences: In the Northeast corner of the complex, four circular stone-covered mounds are arranged in a square. They apparently functioned as a calendar; gaps in the walls were aligned to highlight significant seasonal positions of the rising sun and moon.