The distribution of alluvial plain on the earth is related to geology. During about 4.5 billion years of the earthfs history, there was a large-scale crustal uplift resulting in formation of mountains, which is called orogeny. The orogeny repeated many times at different parts of the earthfs surface. The mountains were subject to weathering and erosion, and became lower, gentler, and, finally, an undulating terrain of continental erosional plains. These erosional plains of the older orogenic belts are the major setting of rain-fed dry crop agriculture. The most recent orogeny is the Alpine Orogeny, which started 200 million years ago, became active since a few 10 million years ago, and still active. It covers Mediterranean, Himalaya, and the circum Pacific ring. Most high mountains existing today are found within this orogenic belt. (Figure 3)