The Wai'anae range is no longer active volcanically and erosional forces begin to wear it down. As the Ko'olau matures, it also begins to erode. Both the Ko'olau and Wai'anae mountain chains develop ridges and valleys due to erosional forces. It is resistant to erosion because of its composition and, even today, it is 1,000 ft. above any other formation on the island of O'ahu. The island sinks and the sea level rises.
Erosion shapes the two mountain ranges into their present-day form.
Much of this loss probably is due to massive landslides into the ocean as well as erosion by water. Water levels recede primarily due to the Pleistocene ice age. Pearl Harbor is being created by this process as it erodes above sea level. On the windward side of the Ko'olau, in the center of the range at the site of an old caldera, Kawainui marsh starts first as an ocean bay. Later it closes off to become a marsh, the largest fresh-water marsh on the islands.