A large population was found in this area when it was visited by Father Kino in 1700, and it is thought that there must have been a great concentration of population in this fertile valley and delta for a long time. In the lower basin of the Colorado River and in the desert area which adjoins it, has been found evidence of ancient people who worked in stone but did not make pottery.[115] A period followed in which more territory was occupied and in which pottery was made. The finding of datable pieces of trade wares in the valley indicates a period of occupation of some 1500 years by people familiar with ceramics.[116] Archaeologists studying the Patayan or Yuman culture encounter many difficulties. The culture seems to be characterized by a great poverty of material remains, possibly because of a greater use of perishable materials which have not been preserved. Also, until Boulder Dam was built, the river overflowed its banks every year and covered the land with a layer of silt, thus burying much evidence of occupation.[17] In western and northwestern Arizona, the portion of this area which lies within the scope of this book, the one group of people which has been more or less definitely assigned to the Patayan culture is known only from the finding of distinctive, brown utility-wares. The main center of this tribe seems to have been in the Colorado River valley below Black Canyon. There are also two other groups of northwestern Arizona which may, or may not, prove to be manifestations of the Patayan pattern. The area below the Grand Canyon and north of the San Francisco Mountains, bounded on the east by the Little Colorado River and on the west by the Grand Wash Cliffs, was occupied between about 700 and 1100 A.D. by a group of people to which the name _Cohonina_ has been applied.[16] These people lived both in deep and in very shallow pit houses with walls made of timber. It has been suggested that the deep pit houses may represent a Sinagua trait and that the near-surface houses were the true Cohonina form. Masonry was used in the construction of some of the deep pit houses and granaries and forts. The latter are large rectangular buildings with thick walls and parapets which were probably loop-holed. The building of such structures would suggest unsettled conditions. Some time after 1100 A.D., masonry pueblos were built.