The year 700 marks the beginning of the Pueblo I phase—the time when disparate groups began to form larger communities in the upland areas on either side of the northern Rio Grande. Pit houses were still in use, but aboveground buildings of clay and sticks were erected alongside them. Increasingly, the pit houses were sacred spaces, chambers in which religious ceremonies were carried out; these are now known as kivas and are still an integral part of pueblo life. The Pueblo II era dates from 850 and is distinguished by the rise of Pueblo Bonito in Chaco Canyon, northwest of Santa Fe , into a full-scale city and perhaps capital of a small state. It was home to an estimated 1,500 people ruled by a religious elite. But Chaco began to crumble abruptly around 1150 (perhaps because of drought, famine, or warfare), marking the beginning of the Pueblo III period, when the people who were to become today’s Puebloans began building their easily defended cliff dwellings—most famously in the Four Corners area, at Mesa Verde, but also farther south, on the Pajarito Plateau in what’s now called Bandelier National Monument , and in Puyé, on Santa Clara Pueblo land. A drought at the end of the 13th century cleared out the Four Corners, marking the start of the Pueblo IV era and provoking the population to consolidate along the Rio Grande in clusters of sometimes more than 1,000 interconnected rooms. These communities dotted the riverbank, drawing their sustenance both from the river water and from the mountains behind them.