Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

Most of the surficial deposits were formed by geologic processes that occurred over and over again, and some of these processes are still going on today. One kind of geologic process is the erosion of rock by glaciers, the transportation of the resulting debris downvalley by the ice, and the eventual deposition of the debris at and beyond the glacier margins. The resulting surficial deposits are referred to collectively as glacial drift. They include rock debris deposited by glacier ice as well as sand and gravel formed by melt water from glaciers. The term till refers to an unsorted mixture of rock debris formed directly by a glacier. Moraines are ridges of till that have been deposited along the sides of a glacier (lateral moraines) or at its front (end moraines).

A second geologic process responsible for many surficial deposits in the park is landsliding. Some large slides of the past became mixed with water as they moved downslope into valleys and formed mudflows. Mudflows are mixtures of water and rock debris of many sizes and during movement resemble wet concrete. Falls of masses of rock from cliffs are also a type of landslide; they range in size from small pieces that accumulate at the base of the cliff and form a sloping talus of angular rock fragments to falls of whole sections of a cliff. Rockfalls that become broken into material of many sizes sometimes move rapidly far downvalley as avalanches of rock debris.

A geologic process that is going on today just as it did in the past is the erosion, transportation, and deposition of rock debris by streams and rivers. The sand and gravel deposits in river bars and terraces that result from this process are called alluvium.

Still another geologic process that has resulted in some widespread but thin surficial deposits is volcanism. Within the last 10,000 years or so, Mount Rainier has repeatedly erupted clouds of pumice that have settled to the ground over much of the eastern part of the park. Pumice is a type of lava that is so full of gas bubbles that it is light enough to float on water. Because of its light weight, pumice can be transported great distances by wind.

Some geologic processes other than those mentioned are responsible for a few of the surficial deposits in the park; these will be discussed as the specific deposits are described.