Geology

From steep glaciated fjords to smoldering volcanoes, the northern Puget Sound exhibits the effects of dynamic geologic processes. Tectonism, the active process of plate tectonics, has been a major force in Washington State. The scenic Cascade Mountains, bordering the eastern side of the study area, and the volcanic Mt. Baker, located in the southwestern side of northern Puget Sound, resulted from the subduction that occurred when one plate slid beneath another. The processes of plate tectonics have been ongoing for at least 250 million years and are responsible for the bedrock geology in northern Puget Sound.1

On a more recent time scale (thousands of years), episodic glaciation has sculpted the surface features of northern Puget Sound. As a result of tectonic plate movements, the geologic history and ongoing plate-tectonic processes produce a lively environment. The environment is subject to varied dynamic geophysical processes, such as earthquakes, volcanic eruptions, coastal and headland erosion, as well as tidal flat, lagoonal and spit deposition.1

Geologic Setting2

Bellingham offers a unique natural setting for the study of earth sciences. The region has a fascinatingly rich geologic history, lying along the zone where oceanic crustal plates collide with the North American continent. The oldest rocks occur in the North Cascade Mountains and along the coastal hills and adjacent San Juan Islands. These sedimentary, igneous, and metamorphic rocks were originally deposited more than 100 million years ago, far to the south of their present location. The motion of the earth's crustal plates has since transported these rocks several thousand miles northward, until they collided with the western edge of North America. The resulting compression caused the rock layers to become highly folded and faulted, producing spectacular mountain peaks.

Prior to the rise of these mountains, a broad swampy river valley covered Northwest Washington. Over a period of about 10 million years, the river system deposited a 15,000 foot thick blanket of sandstone, shale, and coal, forming the thickest sequence of non-marine sediments in North America (the Chuckanut Formation). Other relatively young rocks were formed by the intrusion of magma, producing bodies of granite that contain deposits of gold, silver, and copper.

The modern landscape includes many features that formed during the Ice Age. Only 15,000 years ago the Bellingham area was covered by a mile-thick sheet of ice, and extensive alpine glaciers still occur in the local mountains; outside of Alaska, 80% of the nation's glaciers are found in the North Cascades. During the Ice Age, a period of volcanic activity resulted in the formation of huge ice-covered volcanoes. The nearest of these is Mount Baker, a huge ice-covered peak that produces occasional steam plumes visible from Bellingham. The waters melting from its vast ice field join the swift currents of the Nooksack and Skagit Rivers, reaching the coastal tide flats after a journey of only 40 miles. Here they mingle with the green marine waters that have so deftly sculpted the beaches, cliffs, and islands of Puget Sound.

Geologic Time3

Even geologists, who have opportunity to practice, have difficulty imagining the great length of time needed for geologic processes. We can think about a million years or even a billion years, but we can hardly imagine the countless small events that fill such expanses of time. And it is just such small events – the settling of a sand grain to the ocean bottom, the tumbling of a rock off a thawing, north-facing mountainside, the death of a small snail – that add up to geologic change.

Climbing a Mountain of Geologic Time

An ambitious hiker who climbs from Nooksack Falls, on the North Fork of the Nooksack River, to the top of Hadley Peak, crowning Chowder Ridge, north of Mount Baker, ascends about 5000 feet vertically (about a mile) straight up and climbs layered rocks of the Nooksack Formation spanning an age range of about 50 million years. Using this same scale, if our hardy hiker were to ascend the 400 million years of strata widely represented in the North Cascades National Park, he would have to climb vertically about eight miles. And if he were to climb metaphorically through strata representing the time encompassed by all the North Cascade rocks, including some that are about 1,600 million years old, he would have to climb about 32 miles straight up. In contrast, for the youngest episode of North Cascade history, the growth of the Mount Baker and Glacier Peak volcanoes in less than 1 million years, he would only have to scramble up about 100 metaphorical feet.

The Geologic Time Scale

Nineteenth-century geologists devised a time scale to represent the relative ages of rocks. The age of the Earth was then much debated, with estimates ranging from a biblical 6,000 years to figures on the order of 40 million years, still far short of the currently-accepted age of about 4.6 billion years. Despite disagreements as to the age of the earth, it was still possible in many places for everyone to agree that one layer of sedimentary rock rested on top of another and that the one on top was younger. As a result, geologists were able to create a relative geologic time scale based on such relationships.

No one knew the ages of rocks expressed in years until dating methods using naturally-occurring radioactivity were developed in the twentieth century. The following sections avoid using relative geologic time terms, such as those found in the figure below. Instead, actual dates are given when possible with the relative ages given in parentheses.

References Cited

1. Adapted from Kachemak Bay Ecological Characterization. Kachemak Bay Research Reserve. Copyright ©2000 Alaska Department of Fish and Game. <http://www.csc.noaa.gov/lcr/kachemak/html/ecosys/physical/geomorph.htm>

2. Western Washington University. Geology: Geologic Setting. <http://www.ac.wwu.edu/~geology/about.htm>

3. United States Geological Survey (USGS) Western Earth Processes Team and the National Park Service. North Cascades Geology. <http://wrgis.wr.usgs.gov/docs/parks/noca/nocageol3.html>

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