The emergence of the Olmec civilization has traditionally been dated to around 1600 to 1500 BC. Olmec features first emerged in the city of San Lorenzo Tenochtitlán, fully coalescing around 1400 BC. The rise of civilization was assisted by the local ecology of well-watered alluvial soil, as well as by the transportation network provided by the Coatzacoalcos River basin. This environment encouraged a densely concentrated population, which in turn triggered the rise of an elite class and an associated demand for the production of the symbolic and sophisticated luxury artifacts that define Olmec culture. Many of these luxury artifacts were made from materials such as jade, obsidian, and magnetite, which came from distant locations and suggest that early Olmec elites had access to an extensive trading network in Mesoamerica. The aspect of Olmec culture perhaps most familiar today is their artwork, particularly the Olmec colossal heads. San Lorenzo was situated in the midst of a large agricultural area. San Lorenzo seems to have been largely a ceremonial site, a town without city walls, centered in the midst of a widespread medium-to-large agricultural population. The ceremonial center and attendant buildings could have housed 5,500 while the entire area, including hinterlands, could have reached 13,000. It is thought that while San Lorenzo controlled much or all of the Coatzacoalcos basin, areas to the east (such as the area where La Venta would rise to prominence) and north-northwest (such as the Tuxtla Mountains) were home to independent polities. San Lorenzo was all but abandoned around 900 BC at about the same time that La Venta rose to prominence. A wholesale destruction of many San Lorenzo monuments also occurred circa 950 BC, which may indicate an internal uprising or, less likely, an invasion. The latest thinking, however, is that environmental changes may have been responsible for this shift in Olmec centers, with certain important rivers changing course.