When earth forces lifted the sea floor, the sea disappeared, and for long periods there were mountains where the sea had been. Pressure and heat changed the rocks, especially the sedimentary rocks, which became harder and tougher. Shale changed progressively into slate and phyllite. All of these rocks are found in the Olympic Mountains. The sedimentary rocks and lava flows, originally horizontal on the sea floor, were tilted and folded when uplifted and this is how we see them today. Long periods of erosion have removed thousands of feet of rock and remolded the Olympics into magnificently rugged mountains. Thus, earth forces build mountains and water slowly carries them back to the sea. So it has been since the first rains fell upon the cooling earth. Today only the oldest rocks remain, for these were the bottom layers. The greater part of the Olympic Mountains are made up of these rocks, now mostly slates and hardened sandstones. This includes all the rock inside a horseshoe-shaped line running from the village of Sappho east to Lake Crescent, Lake Mills, and Deer Park, then south to the west side of Mount Constance and the north end of Lake Cushman and then west to Lake Quinault. The horseshoe-shaped rim of the mountains outside this line is mostly basaltic lava. [Illustration: MOUNT ANGELES SHOWS TILTED ROCKS UPHEAVED FROM THE SEA BOTTOM.] Because fossils are scarce in the oldest rocks, geologists are not certain about their age, but they are thought to be about 120 million years old. The rocks in the outer rim of the Olympic Mountains contain more fossils. These have been found in the sandstones, shales, and limestones interbedded with the thick volcanic rocks. Fish teeth, marine clams, snails, algae, wood fragments, and microscopic shells found here represent forms of life that existed 50 to 60 million years ago. Glaciation Other important geological events started about a million years ago. As the climate of the world became colder a great ice sheet formed to the north and moved down across Canada into the United States. There were periods when the climate warmed and the ice retreated. It advanced again when temperatures lowered during tens of thousands of years. The sheet moved southward at least four times during the last million years.