Complex sentence: Archeologists refer to the period starting as the Pueblo I Period; by the early 10th century the large pit-house settlements had been supplanted by modular construction that later served as the foundation for the Ancestral Puebloan great houses.
Split into simpler sentences: Archeologists refer to the period starting as the Pueblo I Period. By the early 10th century the large pit-house settlements had been supplanted by modular construction that later served as the foundation for the Ancestral Puebloan great houses.