The Sumerians moved into the lower part of the Tigris-Euphrates valley about 3000B.C. The Sumerians made many contributions to Mesopotamian culture, some that are still important to people today. They created a 12 month calendar based on the cycles of the moon. They also developed a mathematical system based on the number 60. Though the mathematics of today is not based on sixty, out of this system based on 60 comes the 360 degree circle, the 60-second minute and the 60-minute hour. The Sumerians also developed bronze and were the first to use a metal plow in farming the land. They were probably the first to use the wheel to carry people and goods. But probably the most important achievement was the development of writing. From Sumer comes the oldest writing system. It is known as cuneiform and is made up of wedge shaped marks representing syllables.