It is noteworthy that ancient writers make no reference at all to the Aral Sea. Apropos of this, Reclus (1898) states: "It is difficult to explain how such a vast water area, at present almost as extensive as the Aegean, could have been completely unknown to the ancients, if it had the same dimensions then as it does now. Greek rulers governed the countries between Persia and the large mountain ranges of Central Asia for several centuries. Moreover, Greek-speaking traders and military leaders crossed the Oxus and Jaxartes Rivers, but none of them made any mention of a second sea lying to the east of the Hyrcanian (Caspian) Sea." However, then came the next warm period, with maximum climatic warming in approximately the sixth century A.D., and the Amu Dar'ya and Syr Dar'ya gradually deviated to the right (poleward). Accordingly, the flow of water into the Aral depression increased, and a body of water was formed there which was large enough to command the attention of the people of that time. In actual fact, writers of the early Middle Ages stopped referring to the discharge of the Amu Dar'ya and Syr Dar'ya into the Caspian. Instead, they reported that the two rivers flowed into the Aral Sea. These writers were acquainted with the Aral; they drew it on their maps and discussed navigation on it.
During the third cold period (late Middle Ages) the water-supply situation for the Aral Sea was more or less the same as during the second cold period (antiquity). The Amu Dar'ya and Syr Dar'ya deviated southward, less water flowed into the Aral basin, and if there was a lake in the basin it was small. Reports concerning the Aral once again ceased (not a single European traveler makes any reference to it). Then, with the onset of the present warm period, the two Dar'yas tended to shift to the north again. Accordingly, the flow into the Aral increased and the level rose.
Lake Sarykamysh. This lake lies between the Caspian and Aral Seas, and its geographical position determines the nature of its water supply. Optimum conditions for filling the Sarykamysh basin with water exist when the centrifugal forces of the Earth's rotation are not so great that the Amu Dar'ya, the Syr Dar'ya, and the other rivers to the east and northeast of the basin deviate too far southward. When the rivers deviate to the south, they bypass the Sarykamysh basin and discharge into the Caspian; this was the case, for instance, during the period of Dnieper glaciation (see Figure 8a). However, the centrifugal forces cannot be so small that the rivers deviate too much to the north either, since then they will bypass the Sarykamysh basin and flow into the Aral; this is the situation during the present warm period (see Figure 8d). Optimum conditions evidently existed during the Warm glacial period and during the long periods of climatic cooling which occurred in postglacial times (Figures 8b and 8c).
The foregoing general considerations are confirmed by the following conclusions of Shnitnikov (1959): 1. Three times during the last 4000 to 4500 years the Sarykamysh basin was filled up by the Amu Dar'ya, and during these periods flow took place along the Uzboi channel.
2. During the time interval between the second and third fillings of the basin (that is, in the first millenium A.D.), Lake Sarykamysh dried up completely. Moreover, there is definite evidence that the basin was dry in the period between the first and second fillings as well. At present, too, the basin is dry.
3. All three periods during which the Amu Dar'ya filled the Sarykamysh basin coincide in time with cool, wet phases in the great rhythmic wetness cycles for the continents of the Northern Hemisphere. The times when the Sarykamysh basin was dry and there was no flow along the Uzboi channel, on the other hand, coincided with the long dry phases of the rhythmic wetness cycles.
The Atrek. In its lower reaches this river has in general a westerly course. The water is entirely used up for irrigation and only reaches the Caspian at the time of floods.
During the first warm period (Bronze Age) the Atrek deviated to the right (northward) and watered the so-called Messerian plain. At this time the archaic-Dakhistan culture existed on the plain. V. M. Masson (1954) suggests that the archaic-Dakhistan culture be dated to the first third of the first millenium B.C. and to some unspecified part of the second half of the second millenium B.C.
During the second cold period (antiquity) the river deviated to the left (southward). Consequently, the Messerian plain dried up, the irrigation system stopped functioning, and the archaic-Dakhistan culture went into a decline.
During the second warm period (early Middle Ages) the Atrek deviated to the right (poleward). The Messerian plain then became well watered once more, and settlers began to come there. It was at this time that the so-called Ephtalito-Turkic culture reached its apex (VI-VIII centuries A.D.).
During the third cold period (late Middle Ages) the river deviated to the left, and the Messerian plain became a desert. According to the data of Masson, settled life in the oasis died out completely in the fifteenth century.
During the present warm period the Atrek tends to deviate to the right (poleward). Murzaev (1957) notes that with the approach to the delta the banks gradually recede and become lower. In this region the Atrek flows through lowlands and turns vast areas into swamps; it divides into branches and its channel shifts northward. Thus, Hasan-Kuli Bay is at present being filled up with deposits from the Atrek, in spite of the fact that the Caspian Sea has a low level, which is favorable for downcutting of the river bed.