Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Focus Page #3 -- Changing Climate, Landscapes, and Life Forms of the Pacific Northwest

Introduction

Climate, landscape, plants and animals--these represent the environment of a given time and place. The geologic record shows many changes in the environments of the Pacific Northwest. Places that are now in a cool climate were once much warmer. Parts of the sagebrush country of North Central Washington were once beneath a giant glacier. Places that are now land were once underwater. Rhinoceroses once lived in what is now Eastern Washington. Large numbers of dinosaurs walked the earth in what is now Montana. How do we know?

What is the evidence of changing environments over geological time?

The rock record is a record of past environments. Sedimentary rocks are especially important in giving us information about past climates, past landscapes, and past life forms. Fossils are an obvious source of information about what lived in the past. Fossils also tell us much about the environment of the place the organisms lived. For instance, fossil corals and coral reefs suggest a place that was beneath warm, shallow ocean water. Fossil palm trees suggest a warm, humid climate on land.

Besides fossils, sedimentary rocks carry a lot of other information about past environments. The sizes of clastic sediment grains tell us about the energy of flowing water. The sedimentary structures and sequences allow us to distinguish rivers channels from river flood plains from desert sand dunes from lakes from bays of the ocean, and so on. By putting together evidence from sedimentary rocks across a region, geologists can see where ancient mountain ranges rose and then were eroded away, where lakes formed and then dried up, and so on.

What are some examples of climate change in the Pacific Northwest?

In Montana, Jurassic and Cretaceous sedimentary rocks that contain dinosaur fossils also contain evidence of rivers meandering from distant mountains, across arid or semi-arid landscapes, to a shallow inland sea. Along the ancient coast where the rivers emptied into the shallow sea are zones of coal deposits and fossils of lush, swampy, sub-tropical forests. This area is now the high plain of Central and Eastern Montana, far above sea level, with winters too cold for lush, sub-tropical forests.

Near Bellingham, Seattle and Cle Elum, Washington, sandstones and shales of the Puget Group, which are of Eocene age, contain fossils of plants, including palmetto-leaf plants. These fossils suggest a warm, humid climate with relatively warm winters. Most people who live in Seattle now do not consider the climate moist, warm and humid enough to be called subtropical--the climate is much cooler now.

At Gingko Petrified Forest State Park, near Vantage, Washington, you can find Miocene fossils from a forest that would have needed warmer winters and more rainfall than occur in the area now.

During the glaciations [GLOSS] of the Pleistocene ice age, climates throughout the Pacific Northwest were colder than they are now, even in those places that the glaciers did not cover. There was more rainfall and less evaporation in the intermountain interior, which led to large lakes forming in valleys that had no outlets for rivers. For instance, Great Salt Lake in Utah is a small pond compared to glacial Lake Bonneville, the much deeper and wider lake that covered the area during the peak of the last ice age. Some of the intermountain valleys of Central Oregon had lakes during the ice ages, where it is now desert or semi-arid steppe and dry land.

How have regional landscapes changed in Northwest geology?

Before the Mesozoic Era, most of Washington and Oregon did not even exist yet. Idaho was approximately the coastal zone of older North America, and a broad, gently sloping continental shelf tapered off into the ancient Pacific Ocean. In the Jurassic, the plate tectonics of the Pacific Northwest changed as subduction started occurring offshore. Since then, subduction and other tectonic activity in the Northwest have created one mountain range after another. Several of the mountain ranges of the Pacific Northwest contain layers of rock that used to be layers of sediment on the bottom of the ocean.

Since the Mesozoic Era there have been many more major changes in Pacific Northwest Landscapes. During the Paleocene Epoch an early form of the Cascades Mountains may have been forming, and the coast was in the vicinity of Puget Sound. But there were not yet any Olympic Mountains or Olympic Peninsula to the west--just ocean. The Olympics formed after the Paleocene Epoch.

After the Olympic Mountains accreted and uplifted, the forearc basin between the volcanic arc and the coast range was a zone of rivers working their way to the sea. Apparently, Puget Sound did not form until the center of the forearc basin was scraped and washed out by large glaciers and the high-pressure meltwater coming out of the base of the glaciers during the Pleistocene Epoch. Then the last glacier retreated and sea level rose, inundating the low parts of the sub-glacial troughs and thus forming Puget Sound.

As geological time unfolds, one of the main themes is landscapes gradually changing--ocean sediments becoming mountains, swamps becoming deserts, river valleys becoming bays of ocean water. The changes continue to occur.

What changes in life forms are seen in the geologic record of the Pacific Northwest?

The Pacific Northwest contains its share of fossils that contribute to the worldwide record of how life forms have changed over the course of geologic time. Like other places around the world, the fossil record of the Pacific Northwest changes from sparse fossils of simple, single-celled organisms in the early chapters of Earth history (the Archean and Proterozoic Eons) to a flourishing of many forms of life in the later chapters of earth history (the Phanerozoic Eon, which comprises the Paleozoic, Mesozoic and Cenozoic Eras--we live in the Cenozoic Era). What follows is a simplified sketch of the record of life forms in the fossils of the Pacific Northwest.

The oldest rocks in the greater Pacific Northwest, from the Archean eon, are found in the Rocky Mountain States, including some of the mountains of southwestern Montana. However, because these rocks were buried deeply in the Earth and greatly heated up since they first formed, they do not retain any actual fossils. The next age sequence of rocks in the area is the Belt Supergroup, found in northwestern Montana. The Belt Supergroup contains stromatolites, which are interpreted to be fossil structures that accumulated from generations of simple, single-celled bacteria that performed photosynthesis and formed mats or thin films on the floors of shallow-water bays.

Next comes the Paleozoic Era, from which there are many fossils in various parts of the Pacific Northwest. Most of the fossils are of ocean-dwelling animals, including mollusks of various kinds, trilobites (a type of arthropod that scuttled about the floor of the ocean and became extinct by the end of the Paleozoic) and fishes, the first vertebrates.

The eastern portion of the greater Pacific Northwest, especially Montana, has yielded excellent fossils of dinosaurs, a diverse group of land-dwelling vertebrates that existed during the Mesozoic Era and became extinct at the very end of the Mesozoic.

Fossil plants of the Paleozoic and Mesozoic eras are found in parts of Montana, Wyoming and Idaho. These fossils reveal that lush forests of ancient times were composed entirely of different types of trees than exist in the world today. Conifers like we see in today's forests started becoming common in the early Mesozoic. Flowering plants and deciduous trees did not come into the fossil record until the latter stages of the Mesozoic Era

In the first half of the Cenozoic Era, after the demise of the dinosaurs, mammals became more diversified and abundant. However, they were not the same species of mammals that exist today. In eastern Oregon well-preserved fossils from this and later epochs of the Cenozoic show that the modern horse is apparently related to precursor species that were much smaller and still had three toes rather than a single hoof.

Even as recently as the Miocene epoch, 15 to 20 million years ago, eastern Washington was the home of animals that did not leave any ancestors on this continent, such as the rhinoceros fossil found near Blue Lake in north central Washington.

During the Pleistocene Ice Ages, between 2 million and 10,000 years ago, there were many species of large mammals in the Pacific Northwest that are now extinct. The first human skeletons occur in the fossil record of the Pacific Northwest at about the time these mammals became extinct, roughly 10,000 years ago. The fossil record from the Pleistocene includes wooly mammoths and mastodons, relatives of the elephant that were adapted to cold climates but did not survive the transition to warmer climates at the end of the last ice age.

Web Links

For information on the types of animals that became extinct at the end of the Pleistocene epoch go to Illinois State Museum website at:
http://www.museum.state.il.us/exhibits/larson/ice_age_animals.html

Glossary terms that appear on this page: sedimentary rocks; clastic; sedimentary structure; coal


Geology of the Pacific Northwest
Focus Page #3--Changing Climate, Landscapes, and Life Forms of the Pacific Northwest
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/2/13