During the last glaciation, when glaciers were much larger than they are now, melt-water streams carrying great quantities of sand and gravel built valley floors up to levels tens of feet higher than they are today. Later, as the glaciers grew smaller, the rivers cut down into their valley floors and remnants of the sand and gravel deposits were left standing in benches or terraces along the sides of the valleys. You can see a good example of such a terrace in the Nisqually River valley beyond Ashford, which is 5 miles west of the park. You cross it on the highway that leads to the park. Cuts beneath the terrace reveal deposits of sand, cobbles, and boulders that look the same as those deposits being formed today by melt-water streams. The terrace west of Ashford was formed a little more than 15,000 years ago, when a glacier extended down the Nisqually River valley to the vicinity of Ashford.
Yesterday’s Glaciers
Mount Rainier’s great sprawling cone would seem incomplete without the glistening sheets of ice that descend its flanks. We have reason to believe that the volcano has borne glaciers ever since its origin—sometimes smaller than now, at other times vastly larger. Mount Rainier probably started to grow during the middle part of the Pleistocene Epoch, or Ice Age, which began more than 1 million years ago, but glaciers had covered this part of the mountains even before the volcano appeared. Masses of rock debris formed by ancient glaciers occur beneath lava flows from Mount Rainier on the west side of Mazama Ridge just upslope from Narada Falls, on the north side of Glacier Basin, and at a few other places in the park.
Mount Rainier may have reached its present size by about 75,000 years ago. Since that time great icefields and glaciers have formed at least three times on the slopes of the volcano and in the nearby mountains. During the first two glaciations, ice completely buried the flanks of the volcano and the surrounding mountains, except for the very highest ridges and peaks. These great ice masses slowly flowed down all the valleys that head at Mount Rainier. The glacier in the Cowlitz River valley, for example, extended 65 miles from the volcano and reached a point about 33 miles west of the community of Randle. Deposits of the younger of these two glacial episodes can be seen in cuts along the Mowich Lake Road for a distance of about 1½ miles inside the park boundary. The glacial deposits were originally more widespread, but in most of the park they have been removed by erosion or covered by the deposits of yet younger glaciers.
Extent of glaciers in the Cascade Range near Mount Rainier between about 15,000 and 25,000 years ago. Arrows show the direction of ice movement; solid black represents modern glaciers on Mount Rainier. (Fig. 16)
During the most recent major glaciation of the park, which lasted from roughly 25,000 to 10,000 years ago, ice again sheathed the slopes of the volcano, but glaciers in the nearby mountains were smaller than before. Most of the glaciers originated at the valley heads, where they gouged out countless bowl-shaped bedrock basins called cirques. Many of the basins held lakes after the glaciers disappeared. (See [frontispiece].) Hikers on the trail to the Paradise ice caves cross the floor of a typical cirque at the head of Paradise Valley. From the Sunrise Visitor Center at Yakima Park you can walk a short distance to a point along the crest of the Sourdough Mountains and stand at the rim of a deep north-facing cirque. Ice originating in this cirque and in the cirques adjacent to it moved northward down the valley of Huckleberry Creek at least as far as the park’s north boundary ([fig. 16]).
These glaciers left most valley walls in the park covered with rock debris. Lateral moraines can be seen along the highway at and just east of Ricksecker Point ([fig. 17]). Other glacial deposits are especially well displayed in roadcuts along the north wall of the White River valley.
Lateral moraine of rock debris at Ricksecker Point. It was formed by Nisqually Glacier when the glacier was at least 1,000 feet thick and about 15 miles longer than it is today. (Fig. 17)