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.
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.
At the same time, valley glaciers flowed out of the mountains of British Columbia, joined forces, and formed a piedmont glacier that moved southward into Puget Sound and against the eastern edge of the Olympic Mountains. A lobe of this glacier branched off and flowed westward through Juan de Fuca Strait. This piedmont glacier, at least 3,000 feet thick, rubbed the northern edge of the Olympic Mountains and sent ice fingers up the valleys. It brought granite boulders from the north and dropped them along the way when it melted. Some of these granite boulders have been found near Camp Wilder, 25 miles up the Elwa River Valley, and as high as 3,000 feet on the side of Klahhane Ridge.
As the ice moved west along the northern border of the mountains, it plowed and scraped the deepened and ancient valley that filled with water when the ice melted. This valley contains Lakes Crescent and Sutherland. These and numerous other telltale marks attest to the work of a thick ice sheet.
Approximately 11,000 years have elapsed since the retreat of the last northern ice sheet from Washington.
With the onset of colder climate, valley glaciers also formed in the Olympic Mountains. They flowed from high mountain cirques down the valleys, probably filling the valleys during times of greatest ice volume and becoming thinner and shorter during times of warmer climate. Like the larger ice sheets from the north, the valley glaciers of the mountains must have advanced and retreated periodically. The greatest advance was as much as 25 to 40 miles in the Hoh, Queets, and Quinault Valleys. A terminal moraine left by a glacier dams Quinault Valley and holds the lake behind it.