The present relief features result mainly from erosion of the volcanic mountains by running water and waves combined with changes in elevation of the islands in respect to sea level. There is little evidence for folding of the bedrock, but faulting has been common. The slopes of a lava dome initiate a radial drainage pattern, but in the areas of heavier rainfall the greater erosion there causes the development of deep canyons with amphitheater heads that contrast strongly with the smaller and less incised valleys found on the dry slopes. Both rising and sinking of the land areas have taken place, particularly on Oahu. Elevation of this island has resulted in raised beaches and terraces along the foot of the mountains, whereas the sinking of the land or rising of the ocean has caused the drowning of valleys to form Pearl Harbor and the deposit of silt in valleys like that of lower Manoa.
GROUND WATER SUPPLIES
Ground water is needed for irrigation and domestic use in Hawaii, and its occurrence depends upon the geology. Clay is an impervious material, and where it has been deposited above pervious lavas and gravel beds, and then the whole has sunk by some hundreds of feet, the clay forms a capping to the pervious beds so that fresh water will be retained in them under hydrostatic pressures. Several artesian areas within the limits of Honolulu have this origin and supply great quantities of water; some of it comes from flowing wells, but most of it is pumped. Difficulty arises if more water is pumped out than enters the ground from rain, because then the salt water, on which the fresh water rests, will rise and pollute the supply, sometimes to such an extent that wells have to be abandoned. Dikes of lava that are dense and impervious to water may cross more open volcanic rocks and seal them to hold supplies of ground water perched at quite high elevations in the mountains. When tapped by tunnels and wells driven into the dike complexes, these supplies of water become important sources for irrigation and municipal needs.