SURFICIAL DEPOSITS SHOWN ON THE GEOLOGIC MAP
BOMB-BEARING DEPOSIT IN SOUTH PUYALLUP RIVER VALLEY
A deposit of unsorted and unstratified rock debris that contains many light-olive-gray breadcrust bombs is exposed along the highway in the South Puyallup River valley (fig. 4, locality 23). Breadcrust bombs are formed when blobs of lava are thrown from a volcano; as the bomb cools, a solid skin forms, which then ruptures as gas pressure in the lava causes it to expand slightly. The bomb's exterior is a network of fissures and segments that resemble those on a loaf of hard-crusted bread; inside, most bombs are very porous because of the gas bubbles that formed in the lava (fig. 16). Many bombs have one flat side, which was formed when still-plastic blobs of hot lava struck the ground after flying through the air. Bombs in the South Puyallup River valley are 6 inches to 4 feet in diameter.
The bombs are contained in a deposit of loose purplish gray sand. Although the bombs are the most conspicuous part of the deposit, fragments of dense glassy dark-gray andesite as much as several inches in diameter are also abundant. The deposit is at least 200 feet thick in places, and it overlies very coarse mudflows and alluvium in which no bombs were seen. On top of the deposit are two thin clayey mudflows which probably were formed about 1,000 and 600 years ago, respectively. A carbonized log 4 feet long that was found just above the base of the bomb-bearing deposit had a radiocarbon age of about 2,500 years. Streams have eroded the bomb-bearing deposit to form ridges that greatly resemble lateral moraines near the West Side Road.
The deposit originated when hot rock rubble, ash, and bombs avalanched from the top of the volcano during an eruption. Even though the rock debris flowed down the South Puyallup River valley in a fluid manner, the magnetic properties of rock fragments indicate temperatures of hun dreds of degrees above the boiling point of water when movement stopped. Thus, the rock debris must have flowed in a dry condition, and it probably was "lubricated" by steam, hot air, and other gases.
As many as five mudflows, each of which is from a few feet to several tens of feet thick, lie on low terraces, valley sides, and some ridgetops in and adjacent to the valleys of Tahoma Creek and the North and South Puyallup Rivers (fig. 17). All have been formed within the last 4,000 years, and one is only about 440 years old. The mudflows are un sorted mixtures of boulders of many sizes in a yellowish-orange matrix of sand, silt, and clay. One, exposed in road cuts at Round Pass (fig. 4, locality 24), can be traced along a trail to Lake George to a height of about 350 feet vertically higher than the pass, which indicates that the Tahoma Creek valley temporarily must have had at least 1,000 feet of mud flowing in it. Remnants of the deposit on the sides of the South Puyallup River valley indicate a comparable thickness or depth there. At Indian Henry's Hunting Ground, on the divide between the Tahoma Creek and Kautz Creek valleys, the southwest margin of the mudflow forms an abrupt front about 17 feet high where it is crossed by the Wonderland Trail (fig. 4, locality 25). The boundary there between the mudflow and the adjacent surface is conspicuous because pumice layer Y abruptly disappears as one passes from the older surface onto the mudflow.
The approximate maximum height of the deepest mud flows in the valleys west of the volcano, shown by a heavy dashed line on the geologic map, is reconstructed from the highest known remnants of the deposits; this line probably represents the height attained by a single mudflow that occurred about 2,800 years ago. Another mudflow temporarily reached almost as high on the valley sides about 600 years ago. It had enough volume to move 25 miles downvalley and to extend an additional 15 miles into the Puget Sound lowland. All the mudflows originated in avalanches of clay and altered rock from the west side of the volcano at Sunset Amphitheater (frontispiece).
Last Updated: 01-Mar-2005