These beds of sandstone, shale, and coal make up a sequence of rocks called the Puget Group, which is 10,000 feet thick. Wave-ripple marks and remains of plants show that the rocks were formed in shallow water fairly close to sea level. How could the rocks have piled up to this great thickness? The coastal plain and adjacent basin must have been slowly sinking, and the influx of sand and clay must have just barely kept pace with the downward movements. [Illustration: Mount Rainier] A little less than 40 million years ago, the western Washington landscape changed dramatically. Geologists R. S. Fiske, C. A. Hopson, and A. C. Waters have discovered that volcanoes then rose on the former coastal plain at the site of Mount Rainier National Park and became islands as the area sank beneath the sea. When molten rock was erupted underwater from the submerged flanks of these volcanoes, steam explosions shattered the lava into countless fragments. The resulting debris, mixed with water, flowed as mud across great areas of the submerged basin floor. [Illustration: Outcrop of gray to brown sandstone and dark-gray to black coaly shale in the Puget Group along the Mowich Lake Road. (Fig. 1)] You can see rocks formed from these layers of volcanic mud and sand in cuts along the highway on the east side of Backbone Ridge and between Cayuse Pass and Tipsoo Lake. Look there for alternating beds of grayish-green sandstone and breccia, a concretelike rock in which the pebbles have sharp corners. These rocks are known as the Ohanapecosh Formation. Like the Puget Group, the Ohanapecosh Formation is at least 10,000 feet thick. Yet, nearly all of it accumulated in shallow water as western Washington continued to sink slowly during the volcanic eruptions. The long-continued sinking finally ended after the Ohanapecosh volcanic activity ceased. Western Washington was then lifted several thousand feet above sea level, and the Puget and Ohanapecosh rocks were slowly compressed into a series of broad shallow folds. Before eruptions began again, rivers cut valleys hundreds of feet deep, and weathering of the rocks produced thick red clayey soils similar to those that are forming in some areas of high rainfall and high temperature today. Look for the red rocks formed from these old soils in roadcuts as you drive along the Stevens Canyon road about 2 miles southeast of Box Canyon.