well into the formation of these mountainside valleys, as illustrated by later Pololu lava flows, which covered the north and northwestern end of Kohala volcano and often flowed into Pololu Valley. Recent seafloor mapping seems to show that the valley extends a short way beyond the seashore, then terminates at what may be the headwall of the great landslide found off the northeast coast of Kohala. In addition to being a primary factor in the development of the largest valleys, the dike complex plays another important role —that of creating and maintaining the water table. Hawaiian lava is extremely permeable and porous. Rainwater easily seeps into the lava, creating a large lens of fresh water below the island surface. In most places, the top of the water table is just a few feet from sea level, as the fresh water lens is floating on salt water. Unlike porous lava flows, however, dikes cool underground into dense rock with few cracks and vesicles. The dikes act as impermeable vertical walls through which groundwater cannot flow. Deep layers of ash compact into impermeable layers between lava flows, preventing percolation of the rainwater. Rainwater that would ordinarily seep right through the ground gets trapped in the dike "reservoirs," located well above sea level. The groundwater is confined to a flow along the dike until it finds a means to escape. In Kohala, the numerous dikes near the summit inhibit groundwater from seeping downslope to the northeast, where it naturally wants to go. Rather, the Kohala dike complex guides it northwest or southeast, down the axis of the rift zones, just like the surface water. On the other hand, the three smaller valleys between the large ones - Honopue, Honokea, and Honokane Iki - as well as the many smaller gulches which are not yet valleys, are deprived of groundwater by the orientation of the rift zone and its dikes. Without the large amount of water that is received by the bigger valleys, these valleys grow far more slowly. Due to its topography as essentially a flat crater floor surrounded by cones and fault scarps, the main caldera is affected relatively little by erosion from water. Most of Kohala's summit groundwater ends up in either Waipi'o or Honokane Nui; the enormous amount of water routed through these valleys results in a large amount of water erosion, which causes the valley walls to frequently collapse,