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The top of the zone of flowage consequently determines the lower theoretical limit of the zone of interstitial waters, which extends from that limit to the surface of the earth. As thus defined, depth to the zone of interstitial water is largely a function of rock strength. It has been estimated that, for very strong rocks, the zone of flowage ranges from a depth of about 10,000 metres to 13000 metres from the surface. However, it is possible that subsurface water does not extend to depths as great as those suggested by these figures. It is thought that, on the average, only 37% of the openings in stratified rocks and about 50% of the voids in igneous materials are actually taken up by the water. In stratified rocks, the water is accumulating due to gravity hence, with less water, most of voids are empty. In igneous rocks, water is controlled by capillarity, which is independent of gravity. We are going to discuss these forces in a short while.