As to the origin of the steppes of the Ukraine, scholars differ. Every one of them thinks he has found the only correct explanation. In reality, the origin and preservation of the Ukrainian steppes can be traced to the combined action of various causes. In the first place there is the continental dry climate. The amount of rainfall is too slight for the development of forest-flora; the drought of the summer and fall too long. A minor cause is the salt content of the steppe-soil, which, however, is apparent only in places. On the other hand, the shape of the ground is very important. Where the land is level, where the dry steppe winds have free play and the rainwater cannot easily dissolve and wash away the salt of the soil, the steppe prevails. Where the land is cut by river valleys and balkas, however, there is more shelter from wind, more moisture, and no salt in the soil, [107]so that conditions are given which are favorable for the development of tree vegetation. For this reason not only the valleys of the rivers, but also the balkas, which but seldom carry water, have always had tree growth, and even woods and groves. The trees which are planted there thrive very well, while attempts at cultivation in the real level steppe almost regularly fail. The most important foundation for the existence of steppes, however, is their character as remains of the old post-glacial steppe formation. Since the beginnings of the alluvial epoch, its territory is being won by the forest, which is constantly pushing forward toward the south and southeast, using the river valleys as the main lines of advance. In this advance toward the south, the forest has now been stopped by man before it was able to reach the shore of the Black Sea and the Sea of Azof.