The Andean preceramic refers to the early period of human occupation in the Andean area of South America that preceded the introduction of ceramics. This period is also called pre-ceramic or aceramic. The earliest humans that came to South America are known as Paleo-Indians. This period is generally known as the Lithic stage. After this came the period that is widely known as Archaic, although there are also some different classifications of this period. The precise classification is complicated because somewhat different terminologies tend to be used for North America and Mesoamerica. The Andean preceramic period would include cultures that belong to Lithic and Archaic stages. The Zaña Valley in northern Peru contains the earliest known canals in South America. These were small stone-lined canals which drew water from streams in the Andes Mountains region. These canals may have been built as early as 4700 BC. A great deal of archaeological work has been done in Peru in relation to the preceramic cultures, and while Caral/Norte Chico civilization has now been studied extensively, there are also many other sites being studied. Caral-Supe civilization Caral/Norte Chico is one of the most notable and well-studied cultures in Peruvian archaeology. The most impressive achievement of this civilization was its monumental architecture, including large earthwork platform mounds and sunken circular plazas. Also, these preceramic peoples built massive irrigation and water management projects. Archaeological evidence suggests a very early use of textiles, and in particular the use of cotton. Also, recent studies (2013) indicate that maize played a significant role in this civilization starting as early as 3000 BC, contrary to previous findings. Beans and sweet potato were grown as well. Caral/Norte Chico sites are notable for exceptional collective density, as well as individual size. Haas argues that the density of sites in such a small area is globally unique for a nascent civilization.