Glaciers in Southcentral Alaska

Alaska is well known for glaciers, the great rivers of ice that cascade out of its high mountains. It is easy to see these in Alaska, and marvel at their immense size and power. What is often overlooked, however, is the great changes glaciers wrought across the entire landscape of Southcentral Alaska long before recorded history. Indeed, a mere 18,000 years ago, near the end of the Pleistocene Epoch, virtually the whole of Southcentral Alaska was covered with glaciers. The chances are better than not that the very ground you will walk on today is either glacier-scoured bedrock or debris deposited by an ancient ice sheet.

Where Glaciers Occur

Glaciers form in locations that are cool enough for snow to fall in the winter, but not warm enough in the summer to melt all of the snow.

The coldest parts of Alaska, the North Slope and the Interior do not have any glaciers. Even the Brooks Range only has a few small glaciers. Not only are these areas are far from any sources of moisture that are ice-free in winter, the Gulf of Alaska and Bering Sea, but they are also blocked from these water bodies by mountains. Relatively little snow falls on the North Slope, and it does not take very long to melt completely in the short summer. The Alaskan Interior receives more snow, but it has the warmest summers in Alaska to complement its very cold winters, so again, the snow melts away every summer.

It is actually in the warmer parts of the state, such as Southcentral Alaska where most glaciation occurs, because these areas receive large amounts of snow. This precipitation is due to humid North Pacific air slamming into the high peaks of the Chugach Mountains along the coast. This moist air rises, cools, and is forced to dump its water content as rain or snow. Several dozen feet of snow can accumulate in the ranges of Southcentral Alaska in winter. (Compare the snowfall statistics in windward Valdez and leeward Anchorage in the Precipitation chapter). These coastal areas also have relatively cool summers due to the same humid North Pacific air. In some locations the snow never melts completely. Instead it accumulates and gets deeper over the years until it begins to flow downhill under its own weight. This is what we call a glacier.

Causes of Ice Ages and Glaciations

While still debatable, ice ages are thought to occur primarily due to the positioning of the continents. If continental landmasses are positioned in such a way so as to prevent warm ocean currents from reaching the poles, as they are now, there will be considerable cooling of the polar climates. Less importantly, the presence of mountains can affect the climate by altering the pattern and circulation of air masses.

Glaciations and interglaciations within an ice age are correlated with short-term changes in the amount of solar energy received by Earth. Several variables combine to affect the amount of energy received.

Primarily, the Earth’s position and rotation relative to the sun are not as stable as we tend to think. There are three ways in which they shift over time scales thousands of years in length, which influence the overall climate. First, the shape of our elliptical orbit changes on a 100,000-year cycle, from more ellipsoid to more circular. This alters Earth’s distance to the sun as much as 5%. Secondly, the Earth precesses on a 26,000-year cycle, with its axis pointing in different directions, much like a slowly spinning top. The precession causes the seasons to gradually cycle through the year. The precession combined with the elliptical orbit alters the season during which the Earth is closest to the sun. Thirdly, the angle of tilt of the Earth’s axis changes. While it is now tilted 23.5° relative to the sun’s equator, this angle can vary between 22° and 24° on a 40,000-year cycle. A greater tilt tends to create more extreme climates at the poles.

In addition to these factors, large amounts of ash and dust in the atmosphere can have a similar effect. These could result from volcanic eruptions or meteorite impacts. Varying energy output from the sun over time also influences global climate.

The combination of all or some of these factors has caused warm and cool periods over the millennia. As surely as Alaskan glaciers are retreating today, eventually the climate will cool again and they will return to plow across the landscape, obliterating everything in their path as they have many times before.

Glacial Features on the Landscape

Glacial features can be seen all over Alaska, if you know what to look for. Glacial evidence on the landscape can be divided into two kinds: erosional features that form as glaciers scour the mountains of their origin and depositional features that form where they dump bits and pieces of these mountains across the lowlands.

Erosional Features of Glaciers

As glaciers form at the heads of valleys and begin to erode, they create cirques, which are large bowl-shaped depressions whose back walls are often steep precipices. Cirques can be found at the head of almost every mountain valley in Southcentral Alaska. As cirques erode back into the mountain, they approach other cirques that originated in other valleys on the same mountain. Between them, only a sharp, steep ridge will remain, known as an arête. Commonly, numerous cirques will form on a mountain from all sides, leaving only an extremely steep, pointed peak, known as a horn. After deglaciation, these empty cirques often host small lakes, called tarns.

Rivers generally erode downward in an alpine environment, creating a V-shaped valley. Glaciers, on the other hand, fill valleys and erode all sides, forming a U-shape. U-shaped valleys can be observed throughout Southcentral Alaska. In higher mountain valleys, streams have begun to downcut through the glacial deposits, creating a small V-shape in the bottom of the larger U-shape.

Hanging valleys are created when a smaller tributary glacier merges with a larger glacier. Because the larger glacier has so much more erosive power, its valley will be much deeper than that of the tributary glacier. After the glaciers recede, the small tributary valley enters the major valley atop a high cliff, down which waterfalls often tumble.

Erosional Glacial Features

The deeply carved Kenai Mountains around Seward are filled with post-glacial features, many of which are now flooded by sea levels that have risen in the last 10,000 years. Isostatic rebound may one day raise some of these fjords and drowned cirques out of the sea.

Fjords develop when seawater encroaches into a deglaciated valley. During a glacial period, two phenomena occur that contribute to the creation of fjords. First, sea levels are lowered as water is locked up in ice. Glaciers can form in these dry valleys. Secondly, the tremendous weight of the glaciers can depress the land, as much as 600 feet in some places. When the glacial period ends these deepened valleys are flooded as sea level rises. Through a process known as isostatic rebound the depressed surface rises very slowly to its original elevation, even lifting the land back above sea level at some locations. Isostatic rebound can take thousands of years, which is why Alaska still has many fjords. In Southcentral Alaska, large fjords are found in Prince William Sound and Kenai Fjords National Park. Turnagain Arm, south of Anchorage, is also a fjord.

Glaciers smooth the underlying bedrock by scouring. In Southcentral Alaska, many of the lower mountain peaks were completely covered by glaciers and therefore have a smooth, rounded appearance. The higher peaks, on the other hand, may never have been covered by glaciers, and maintain a rougher, jagged appearance which indicates that the predominant erosive force has been the freezing and thawing of water. This pattern can easily be seen in the Chugach Mountains near Anchorage. Also notice the smooth appearance of Mount Susitna (Sleeping Lady) across Knik Arm from Anchorage. This mountain was completely covered by glaciers in the past.

A related feature, the roche mountonnee, a glacially plucked hill, can also be seen at some places. A glacier may ride over a small knob of bedrock in a mountainous region. When it does this, it erodes and smooths the upstream side of the hill, and plucks rocks out of the downstream side of the hill. The end result is a hill that has a shallow slope on one side and a steep one on the other. Bodenburg Butte, just south of Palmer (the 900-foot-high bump between the highway and the mountains as seen from the Knik River Flats), is an example of this.

At many places in the bedrock, large grooves are left behind. Boulders carried by the glacier gouge the underlying rock with tremendous pressure and scrape the material away. Some of these are clearly evident on the top of Bodenburg Butte and can be seen in many places on the exposed bedrock of other mountains in Southcentral Alaska. Striations are linear scratches that are left in rocks as a result of rubbing against other rocks under the immense pressure of glacial ice, some good examples of these can be found in the parking lot of McHugh Creek, near Anchorage.

Depositional Features of Glaciers

Whatever glaciers remove from one place, they deposit somewhere else, producing unique formations across the landscape. In general, debris left by glaciers is known as glacial drift. Drift deposited directly out of the ice, without the influence of flowing water is called till. Glacial till can be differentiated from water-deposited sediment because till is unsorted. Water-deposited sediments are characterized by sorted layers of alternating larger particles, which were left in times of relatively high velocity, and smaller particles, which were deposited during times of relatively low velocity. Till is deposited when glaciers simply melt and dump everything from silt to boulders in the same place at the same time.

Glaciers can dump very large boulders on an otherwise flat landscape. If it is obvious that these boulders could not have been carried by a river or fallen down a mountain slope, it is highly likely that a glacier deposited them. These out-of-place rocks are known as glacial erratics. Along the sides of the Chugach Mountains behind Anchorage are some large boulders of granite. Because the nearby Chugach Mountains contain no granite themselves, these rocks had to have been transported from the Talkeetna Mountains or the Alaska Range, where granite is common.

One of the most common depositional features is the moraine. These are linear ridges of glacial till that can be hundreds of feet high and many miles long. While glaciers flow they continually carry till with them. If the snout of the glacier terminates in the same place for a long period of time, all the material carried by the glacier is deposited in this location, forming what is known as a terminal moraine. If the glacier advances later, the terminal moraine will be overridden and largely obliterated. Therefore terminal moraines usually indicate the farthest extent of glaciation during a given ice age. Lateral moraines occur parallel to the direction of glacial flow. These form when large amounts of rock and silt are scraped off or fall from valley sides and are carried along the edge of the glacier rather than in the middle. If the glacier melts, this debris forms long, low ridges along the sides or the base of mountains.

Depositional Glacial Features

Retreating glaciers in the late Pleistocene deposited the rolling hills and lakes of the Upper Cook Inlet Basin. The dominant feature here is the Elmendorf Moraine, represented by the piles of drift deposited by a large, retreating glacier during the end of the last glaciation (the Naptowne Glaciation).

Moraines are ubiquitous in Alaska. One of the most prominent is the Elmendorf Moraine, left by the Naptowne glaciation. This moraine can clearly be seen from the Captain Cook Monument in downtown Anchorage. It is the large long hill north of Elmendorf Air Force Base. The hill stops at the edge of Knik Arm and then continues to the west on the other side of the water. To the east it stretches almost to Eagle River. It was deposited by ice originating in the Knik and Matanuska Valleys. On the Kenai Peninsula, the moraines that contain Skilak and Tustumena Lakes also date from the Naptowne Glaciation. The material from these moraines originated in the Kenai Mountains. The highway crosses the moraines that surround Skilak Lake just east of Sterling on the Sterling Highway. Just southwest of Soldotna the highway crosses another moraine that dates from the Naptowne Glaciation. Interestingly, glaciers that originated in the Alaska Range on west side of Cook Inlet, rather than in the Kenai Mountains deposited the material. The Homer Spit is a glacial moraine that formed at the mouth of Kachemak Bay. The original moraine has been added to by beach sediment.

Eskers are another long, sinuous feature of glaciers. Many glaciers have streams flowing under them. These streams melt the ice lying above them and in doing so release rocks that have been trapped in the glacier. These rocks fall to the streambed and accumulate over the years as the flowing glacier continually replenishes the supply. Upon the final retreat of the glacier, the pile of rocks that made up the stream bed is left behind, forming narrow snake-like ridges upon the landscape, which can be dozens of feet high and miles long. Numerous eskers occur east of Palmer along the Glenn Highway south of Moose Creek, although heavy vegetation makes them difficult to discern. More obvious ones occur along the Denali Highway near the Tangle Lakes.

Drumlins are a less well-understood form of glacial deposition. These are large hummocks, the shape of “the bowl of an inverted spoon” that were deposited on the glacial plain, in groups of dozens or hundreds. They can be several hundred feet high and up to a mile long. One theory of their origin is that they are material deposited in earlier glaciations that was reworked by a subsequent glaciation. Several drumlins are in east Anchorage, both Russian Jack Springs Park and the Alaska Pacific University campus are situated on drumlins.

Lakes are omnipresent in recently glaciated plains. Most of these are kettle lakes. When glacial ice is retreating, large blocks of ice may be left behind. These melt slowly and in the meantime gravel is deposited around them from the glacial streams that flow across the landscape. The gravel can insulate the ice blocks and slow the rate of melting. When the ice has finally melted away, a depression remains, which fills with water to create the kettle lake. Nearly all of the small lowland lakes in Southcentral Alaska are kettle lakes. Other lakes are formed when glaciers scour and deepen the land beneath them. If they leave a terminal moraine at the end of one of these deepened locations, a lake can result. Portage Lake as well as Skilak Lake and Tustumena Lake on the Kenai Peninsula are examples of this. Moraines can also block mountain valleys creating lakes such as Kenai Lake and Eklutna Lake.

Glacial History of Southcentral Alaska

Glacial records are difficult to piece together because glacial deposits from one glaciation can be obliterated by a subsequent glaciation if the more recent one extends farther than the previous one. Not only that, but glaciers continually advance and retreat in response to relatively small changes in the local climate. Even within a major glaciation significant retreats and re-advances can occur at different times in different places on time scales ranging from dozens of years to thousands of years. Alaskan glacial geologists have put together some major pieces of Southcentral Alaskan glacial history. Alaska experienced numerous glaciations in the past, and also numerous interglacial periods, including the one in which we currently live.

Glaciations in the Late Pliocene

The current Ice Age, the Late Cenozoic Ice Age, began at the end of the Pliocene Epoch, about 2.5 million years ago. Although Southcentral Alaska experienced numerous glaciations during this time, the record is very unclear as subsequent glaciations have eliminated much of the evidence that was deposited during these earlier glaciations. The Late Cenozoic Ice Age was the first major ice age on the planet since the Permian Period, 275 million years ago.

Glaciations in the Early to Middle Pleistocene

The glacial record from this period is likewise sketchy. There is no question that there were glaciations during this time, and several of them were named in the 1950’s (the Mount Susitna, Caribou Hills, and Eklutna glaciations). In the last two decades, however, doubt has been cast on the dates and number of glaciations during this time period. What can be said with relative certainty is that some glaciations during this time extended farther than any glaciation that has since occurred.

A more recent glaciation seems to have occurred sometime before 125,000 years ago. Earlier evidence had indicated that this glaciation, which was referred to as the Knik Glaciation, occurred sometime between 53,000 and 75,000 years ago, which would place it in the Late Pleistocene, but more recent evidence has pushed the date back. During this time there may have been advances in some locations in Southcentral Alaska, but not in all locations, which could explain some of the discrepancies in the data.

Glaciations in the Late Pleistocene

After an interglacial period, the ice again returned 24,000 years ago as the well-documented Naptowne Glaciation, which lasted until about 12,000 years ago. This glaciation was not as extensive as the previous ones. The Naptowne corresponds with the North American Late Wisconsin Glaciation. There were at least four major glacial advances and retreats during the Naptowne.

It was during the Naptowne Glaciation that a major lake known as Glacial Lake Ahtna formed in the Copper River Basin. As glaciers clogged the mountain valleys on all sides, meltwater accumulated in the unglaciated basin, forming an enormous lake. The lake water intermittently flowed northward through Mentasta Pass into the Tanana River and northwestward to the Susitna River. Most of the water from the basin now flows southward, through the Copper River into the Gulf of Alaska. Lake deposits from Glacial Lake Ahtna are as much as 130 feet thick in some places. The lake seems to have been a fairly consistent feature of the landscape until about 10,000 years ago.

Quaternary Glaciation in Southcentral Alaska

150,000 years ago glaciers entirely covered Southcentral Alaska (all green areas). These retreated, but later readvanced, leaving just a few low-lying areas uncovered (light green) at their maximum, 16,000 years ago. Great glacial lakes periodically developed in these uncovered areas. Glaciers now inhabit only a few regions (dark green).

One or more glacially dammed lakes also evidently formed on the Kenai Peninsula in a region stretching from Sterling to south of Tustumena Lake during the Naptowne Glaciation.

It was also during the Naptowne Glaciation that the Bootlegger Cove Formation was deposited in the Anchorage area. Much of this formation is clay and fine sand, which date to about 16,000 years ago. These fine materials were deposited in a marine environment, probably from the estuary of a major glacial river. These deposits held water within them, which gave them the unfortunate characteristic of liquefying when shaken, which is exactly what happened during the 1964 earthquake. Sections of the west Anchorage neighborhood of Turnagain slid into Knik Arm when the underlying deposits liquefied. Earthquake Park was established in the area of devastation.

The Elmendorf Moraine, which is about 14,000 years old, represents the final major glacial advance of the Naptowne Glaciation. We know that there had been a significant glacial retreat prior to the deposition of this moraine because it lays on top of the Bootlegger Cove Formation, which was deposited when the area was covered with water, not ice. Before sea level rose to its current position, the ancestral Knik/Matanuska River removed part of the moraine where Knik Arm now lies.

Ice Advances in the Holocene

Several ice advances and retreats have occurred since the beginning of the Holocene Epoch, although none would qualify as a true glaciation. The glaciers of Southcentral Alaska actually were at a minimum extent between 9,000 and 6,000 years ago during the Hypsithermal Interval, when average summer temperatures were up to 6° F warmer than those today. Following the Hypsithermal Interval we entered a period known as the Neoglaciation, in which a cooler climate led to glacial advances. The present warming trend may be ending the Neoglaciation, or it may be a brief aberration.

On an even smaller scale, a Medieval Warm Period began in about 800 A.D., which was followed by a global cooling in about 1300 A.D. This period is known as the Little Ice Age, which ended in the mid-1800s. This period of cooling led to slight glacial advances. A local example of this is that the early 1800s Portage Glacier reached farther down Turnagain Arm than it had in the last 5000 years. The Little Ice Age may also have caused the disappearance of Viking settlements from Greenland. In the last 150 years global temperatures have been rising dramatically and most glaciers have been retreating in Southcentral Alaska.