The purpose of ground control is to create a stable rock structure that can withstand the stresses that nature applies to it. Traditionally, coal mine operators have been primarily concerned with stress caused by the weight of the rock itself. The pillars however, carry most of the vertical gravity load from the overburden, leaving only the weight of the immediate roof to be carried by the supports. In the early 1970's, researchers began to actually measure stresses underground. They made a surprising discovery-in nearly every case, the horizontal stress was greater than the vertical stress, often several times greater. Although this unexpected finding caused quite a bit of head scratching, it explained many ground control phenomena that were commonly observed underground, such as cutter roof and long-running roof falls. The mystery was finally solved by the theory of plate tectonics (sometimes called continental drift). Plate tectonics explains that the earth's crust is segmented into enormous rigid plates. Earthquakes, volcanism, and mountain building are caused when the plates collide with one another, generating tremendous forces that are transmitted thousands of miles through the earth's crust. Geophysicists have created a World Stress Map based on their current understanding of plate tectonics. After studying all the available stress measurement data, the U.S. Bureau of Mines (now NIOSH) confirmed the relevance of the World Stress Map to U.S. coal mines. Some of the most significant conclusions were: 1. In the eastern United States, the greatest horizontal stresses are usually ENE; 2. There is no clear major direction for horizontal stress in the western United States; 3. The horizontal stresses in the eastern United States are usually 2 to 3 times as great as the vertica1 stresses; 4. Horizontal stresses in the west are generally about equal to the vertical stresses, though some western mines have encountered very high stresses; and, 5. In general, the deeper the mine, the greater the horizontal stress. It is now widely accepted that horizontal stress is nearly always present underground. The only likely exceptions are some small hilltop drift mines. Studies have also shown that: 1. Weak rocks, particularly those containing many weak bedding planes or laminations are more likely to be damaged by horizontal stress than strong massive rocks; and, 2. Stream valleys can concentrate horizontal stresses, and also re-direct them.