A Brief History

During the Pleistocene, continental glaciers covered much of Canada, Alaska, and the northern edge of the continental United States (Figure 6.12). Besides carving vast sections of the northern landscape and depositing huge quantities of sediment in low-lying areas, their impact was felt farther south as glacial outburst floods and winds laden with glacial loess reached deep into Washington and Oregon. Furthermore, in the colder climate of the Pleistocene, large ice caps mantled mountain ranges as far south as Central California, while large freshwater lakes flooded a number of present-day desert valleys in Nevada, California, and Oregon.

Figure 6.12: Extent of glaciation over North America at the Last Glacial Maximum.

Figure 6.12: Extent of glaciation over North America at the Last Glacial Maximum.

As the last Pleistocene ice age came to a close, both the Cordilleran Ice Sheet and the alpine ice caps to the south retreated, leaving behind rugged mountain ranges, deep glacial valleys and bays, and plains covered with thick deposits of glacial sediment. Equally important was the impact of glacial retreat on coastlines. As global ice diminished, sea level rose, radically altering the location and character of the Western coasts.

The time from the end of the Pleistocene to now is regarded as an interglacial period (a warm spell with diminished glaciers), but it has not been without its minor ice ages. The most recent of these, the Little Ice Age, began somewhere between 1300 and 1500 CE and ended by the late 19th century. Presently, the continental ice sheets and ice caps of the Pleistocene are gone, but some 150,000 alpine glaciers remain worldwide, and the impact of the ancient ice sheets and caps can be seen in nearly every region of the Western States.

Alpine Glaciers in the Western States

During the late Pleistocene, alpine glaciers could be found in all seven regions of the Western States, including Hawai’i. In several cases, these glaciers coalesced into ice caps covering entire mountain ranges. In other instances, they merged with advancing continental ice sheets, eventually becoming indistinguishable as separate glaciers, only to regain their distinctiveness as the ice sheets retreated.

Today, alpine glaciers are found largely in the mountains of Alaska and some of the higher ranges in Oregon, Washington, and California. Although the majority of the remaining glaciers tend to be small cirque glaciers or larger valley glaciers, a few large ice caps remain in southeastern Alaska. Modern glacier settings range from non-volcanic mountains like the Olympics and Sierra Nevada to active volcanic peaks found in the Cascade Range and the Aleutian Islands. As one moves farther south in the Western States, the glaciers tend to get smaller and higher. For instance, in southeastern Alaska, large outlet glaciers flow out of the massive Juneau Ice Field into the Pacific Ocean through a network of majestic fjords (Figure 6.13). In contrast, in the Sierra Nevada, a collection of small cirque glaciers perch precariously in mountains at well over 2700 meters (9000 feet) above sea level.

Figure 6.13: Mendenhall Glacier, flowing from the Juneau Ice Field.

Figure 6.13: Mendenhall Glacier, flowing from the Juneau Ice Field.

One of the hallmarks of the alpine glaciers of the Western States is the rugged mountain terrain they carve. As these glaciers retreat, they not only expose characteristic U-shaped valleys, but they also reveal a diverse collection of peaks, bowls, ridges, and lakes scraped into bedrock. For instance, in the North Cascades of Washington and in the Wallowa Mountains of Oregon, glaciers have carved a series of horns, arêtes, and cirques (Figure 6.14). Below these prominent features, we often find chains of lakes that form as meltwater ponds up behind lateral and terminal moraines. Likewise, in the Yosemite Valley of the Sierra Nevada, a network of small Pleistocene glaciers merged like streams flowing into a large river. As the glaciers retreated, they left behind a collection of smaller U-shaped valleys (known as hanging valleys) that drop abruptly into a much larger valley. This is the principal reason that such spectacular waterfalls are found in the Yosemite Valley (Figure 6.15).

Figure 6.14: The glacially sculpted features of the Wallowa Mountains in Oregon.

Figure 6.14: The glacially sculpted features of the Wallowa Mountains in Oregon.

Figure 6.15:  Upper Yosemite Falls in California plunges 440 meters (1420 feet) from a hanging valley into the Yosemite chasm.

Figure 6.15: Upper Yosemite Falls in California plunges 440 meters (1420 feet) from a hanging valley into the Yosemite chasm.

Glaciers also impact landscapes as they melt and deposit sediment, creating features such as Lake Wallowa in the Blue Mountains of northeastern Oregon. Lake Wallowa formed when a large glacier emptied into a broad basin and produced a high moraine that completely surrounded the lower part of the glacier. As the glacier retreated, the depression behind the moraine filled with meltwater, forming the present-day lake.

Among the most dramatic glacial deposits found in the Western States are those produced by glacial outburst floods. These floods occur when a pocket of meltwater that has developed underneath a glacier suddenly breaks through the restraining ice. As the torrent rushes downhill, it picks up rock, sediment, vegetation, and any other debris it can carry, forming a fast-moving slurry that is a significant hazard for many mountain communities. With a number of glaciers in the Cascades and Alaska situated on steep, debris-covered volcanoes, glacial outburst floods are all too common.

Glaciers and Deserts

The arid climate of the Basin and Range and Columbia Plateau may seem like an unlikely place to study glaciers, but even these regions did not escape the influence of the last ice age. The cooler climate of the late Pleistocene glaciation allowed ice caps to mantle entire mountain ranges, while meltwater ponded in basins between these ranges to form numerous, large, shallow, pluvial lakes, such as Lake Bonneville, which covered much of the Basin and Range.

As the climate warmed, the ice caps disappeared, leaving in their wake a multitude of cirques and U-shaped valleys. At the base of the mountains, moraines and deposits of till testify to the lower extent of the glaciers, while glacial drift deposits and dry waterfalls reflect the power of the meltwater streams issuing from them. The pluvial lakes are now gone, leaving behind salt flats and pockets of fine-grained lake sediment buried under the coarser desert sediment. In many of the West’s deserts, wave-cut benches that resemble bathtub rings are visible around the upper perimeter of the basins.

Continental Glaciation in the Western States

Continental glaciers are now only a geologic memory in the Western States, yet the picture was much different during the Pleistocene. Nearly 20,000 years ago, the Cordilleran Ice Sheet covered nearly all of western Canada and parts of the United States. In Alaska, it covered only the southern half of the state, a result of weather patterns that drove snow-laden storms away from the state’s northern section. In northern Washington, outlet glaciers and ice lobes reached down through the Okanogan Highlands and Puget Sound before spilling out onto the lowlands of the Columbia Plateau and southern portions of the Sound. In the vicinity of the North Cascade and Olympic ranges, the outlet glaciers merged with the ice caps that had formed over the range.

One result of the Pleistocene glaciations were huge, U-shaped valleys in the Okanogan Highlands, North Cascades, and Olympics that were carved by outlet glaciers of the Cordilleran Ice Sheet. In Puget Sound, a large lobe, along with the meltwater draining it, sculpted a complex system of fjords while dumping masses of drift nearly 300 meters (1000 feet) thick. This flowing ice and water also sculpted hundreds of drumlins and bedrock hills into linear shapes that marked the flow direction of the ice. Farther to the east, in the Columbia Plateau, the deposits were sculpted into snake-like ridges called eskers. These are believed to be the result of streams flowing through tunnels at the bottom of the glacier that filled up with sediment. In the northern section of the plateau, house-sized boulders (erratics) left behind by the receding ice force farmers to creatively plow their fields. In other parts of the plateau, silt blown from the outwash plains (loess) was deposited well over a hundred miles south of the ice sheet terminus.

Another way the Cordilleran Ice Sheet shaped the Columbia Plateau was by numerous catastrophic floods emanating from the Pleistocene-age glacial Lake Missoula. These flows, sometimes called the Bretz Floods after the geologist who discovered them, would periodically race across the plateau, scouring bedrock and transporting massive amounts of sediment and rock to the Willamette Valley and beyond. The cause of the floods is commonly attributed to meltwater lakes dammed by glacial ice that drained as their ice dams abruptly failed. During the Pleistocene, similar floods occurred in east and south central Alaska and in the Sierra Nevada.

Glaciers and Coastlines

Glaciers have shaped the coastlines of the Western States in three ways. In Alaska, where they reach to the sea, outlet glaciers carve intricate systems of fjords, creating exceedingly rugged coastlines. Since many of these glaciers flow into the ocean, they lose mass by calving. In this way, they create countless icebergs that deposit a chaotic mix of rock and sediment on the seafloor as they melt. Though these sediments are not readily visible in Alaska, they are common along the coast of Washington.

Another way that glaciers have shaped the West Coast is through the rise and fall of sea level. Approximately 20,000 years ago, the coast of Oregon, Washington, and California was 65 to 130 kilometers (40 to 80 miles) farther west than it is now, since sea level was nearly 90 meters (300 feet) lower. At that time, the coast was most likely a broad plain that gradually rose to the base of the present-day coastal mountain ranges. As the ice sheet retreated, increased meltwater flowed back into the sea. As the sea rose, the shoreline advanced eastward towards the mountain ranges, eventually resulting in a shoreline characterized by rocky headlands, small beaches, and large estuaries. As present-day glaciers diminish in size and sea level continues to rise due to changes in global climate, several factors complicate our predictions of how the coastlines of the Western States will change. Chief among these is the slow rise of the crust at the western edge of North America due to plate tectonics and subduction. Another significant influence is the continuing slow rebound of the northern half of North America, brought about by the disappearance of the great Pleistocene ice sheets. The degree to which these factors will offset rising sea levels is both a practical and theoretical concern, given the vast number of people living on or near the coastline of the Western States.

Glaciers in Hawai’i

Since Hawai’i is a tropical volcanic island, most people do not think about glaciers or snow ever having existed there. However, glacial deposits—moraines and till—beginning from approximately 70,000 years ago are preserved on the flanks of Mauna Kea volcano. Similar deposits may have once existed on Mauna Loa as well, but they have since been covered by lava flows. Due to the immense height of these volcanoes, snowfall still accumulates on both Mauna Kea and Mauna Loa in winter (Figure 6.16).

Figure 6.16: Snowfall on the peaks of Mauna Kea in winter.

Figure 6.16: Snowfall on the peaks of Mauna Kea in winter.