As is well known, much of the coast region of the country which was occupied by the empire of the ancient Peruvians is arid, as the prevailing winds are from the east and the moisture is taken from them by the high Andes mountains, and thus the rivers are few and with but little water in them, often dry for a long time, and there is but little rainfall, if at all. In order to overcome this lack of moisture and to add to the fertile area of the country, the Peruvians built reservoirs and aqueducts. These aqueducts were sometimes above ground and sometimes underground and some were quite long, one at least having been between four and five hundred miles in length and twelve feet deep and wide. As they did not use the arch they could not well cross depressions or streams and had to make long detours, and sometimes they tunneled in the mountains for the passages. They had sluices by means of which the water could be turned on to the land and they were very careful in allowing the right amount of water to be used on the fields. The construction of these aqueducts, and other great buildings, is all the more remarkable when it is considered that these people had no iron tools of any kind whatever. They were acquainted with the principle of fertilizing the crop. A small sardine-like fish was abundant along the coast, which they caught and often placed with the grains of maize in the holes in planting. Guano, the deposit of sea-fowls on the islands along the coast, was used in great quantity by them, and much care was given to its proper distribution to the different districts. No one was allowed on the islands during the breeding season and such trespass and killing the birds at any time were punished by death. "Inland other forms of manure were used."[20] As most of the country of the Peruvian empire lay in the torrid zone and consisted of low coast land, elevated land, and mountainous regions, it would have all ranges of climate, according to elevation, and without any great changes of temperature in any one part. Thus all kinds of agricultural products could be cultivated, such as would be found in tropical and temperate regions. In the warm coast region cotton was grown in great abundance and the banana and the cassava; in the more elevated regions the great staple produced was maize and also manioc, guava, groundnuts, tobacco, beans, gourds, and tomatoes; and in the higher and colder parts were found the potato, quinoa, coca, and maguey.