The Geology of Baxter State Park and Mt. Katahdin

Features produced by glacial deposition

An active glacier is not composed wholly of ice and snow, but contains a great deal of rock fragments, sand, and mud, especially near the base of the glacier. This material may be deposited directly by a glacier, left scattered about the countryside after a glacier melts, or be reworked by streams formed by melting ice and deposited as sand and gravel. All types of glacial deposits are called glacial drift, a term handed down from the 18th and early 19th centuries, when glacial deposits were considered to be the result of icebergs drifting in the Biblical flood. There are two types of drift deposits: unstratified drift, or material deposited directly by a glacier, and stratified drift, or sediments washed from glaciers into meltwater streams and deposited as sand and gravel. The terms "stratified" and "unstratified" refer to the presence or absence of layering in the sediments, as discussed previously in the section dealing with sedimentary rocks. Stratified or layered glacial sediments have been in part transported and deposited by water.

Unstratified drift deposits

Erratics

Near the summit of Baxter Peak are boulders of sandstone, shale, and other rock types different from Katahdin granite, the underlying bedrock. These rock fragments are called glacial erratics and are the principal evidence indicating that Mt. Katahdin was once covered by a glacier. If the possibility that some energetic climber carried these rocks to the summit is eliminated, there is no conceivable way, other than by means of a glacier, that these rocks could get to the summit of Baxter Peak. The term erratic is usually applied to rock fragments lying free at the surface, but is also applicable to any foreign material, in gravel deposits and the like. Some erratics, composed of distinctive rock types whose bedrock source is known, indicate glacial transport of many miles, even several hundred miles in extreme cases. For example, some erratics on Mt. Katahdin are similar to bedrock which occurs in Canada north of the St. Lawrence River.

Till

The most widespread type of glacial deposit is till, a sediment popularly called hardpan. Till is composed of all sizes of rock fragments, from mud to boulders, which are carried in the basal part of a glacier. As a glacier moves its great weight of ice, it may plaster this unsorted material onto the underlying bedrock to the extent that it becomes firmly compacted, especially if the till contains a great deal of fine particles.

Excellent exposures of till occur in the vicinity of the South Branch Pond campground in the banks of streams in that area. Some of the most easily accessible exposures are along South Branch Pond Brook where till overlies many of the bedrock outcrops of rhyolite and sedimentary rock (see Figure 5). At a point where the auto road is very close to the stream, about one-half mile north of South Branch Pond campsite, the high bank between the road and the stream exposes more than 50 feet of till. In fresh exposures this till is blue-gray in color and so firmly packed that it is nearly impossible to dig. Most of the larger rock fragments in this till are sandstone and shale, many containing fossils, although there are fragments of Traveler rhyolite near the base of the till.

The map of glacial geology (Plate 1B) shows the principal till deposits in Baxter State Park.

Moraines

A moraine is a ridge of bouldery material which accumulates at the end or temporary margin of a glacier. As mentioned previously, the deposition of a moraine occurs when the rate of ice melting at the terminus equals the rate of forward motion of a glacier. The actual terminus of the glacier is stationary; the ice within the glacier, however, is continually moving. As a result, fragments of rock are carried by the inner ice down to the terminus and deposited there. In areas of valley glacier development in a given region, several moraines may be formed at the end of a valley glacier at the same time if climatic conditions are the same throughout the region. A warming trend causes a glacier to retreat from the position marked by its terminal moraine, with retreat continuing as long as warm weather prevails. After the development of one series of moraines, a cooling trend causing the glacier to advance once again, will override and probably destroy any previously formed moraines. Most moraines record only a general warming period which is punctuated with short periods of stable climate.

The most prominent moraine in Baxter State Park is the irregular ridge east of Basin Ponds on the Chimney Pond Trail, which is over 50 feet high and more than 2 miles long. The Basin Ponds owe their existence to this moraine, the ponds having formed in the depressions behind the moraine. Part of this moraine is visible in Figure 12.

Figure 12

Figure 13

Valley glaciers flowed from South Basin, Great Basin, North Basin and Little North Basin and merged along the line marked by the Basin Pond moraine. Following the deposition of the Basin Pond moraine, these glaciers retreated for approximately one mile and the North Basin glacier became separated from the combined glacier coming from Great Basin and South Basin. Smaller moraines in North Basin at Blueberry Knoll and in Great Basin at Dry Pond on the Chimney Pond Trail record a halt in the general retreat from the Basin Pond moraine. The typical hummocky morainic topography near Blueberry Knoll is shown in Figure 13.

At the very head of North Basin there is an irregular deposit of boulders which records the position of either a very small glacier or more likely, a permanent snowbank, over which boulders falling from the headwall above rolled (see Figure 11A).

Ground moraine

Deposits of unstratified drift marked by irregular topography but lacking pronounced linear features such as end moraines, are termed ground moraine. Ground moraine occurs in nearly all of the lowland area south of Mt. Katahdin, in several of the large valleys in the central part of Baxter State Park, and on the floors of the cirques (see Plate 1B). These deposits have a similar appearance, although they were likely formed in different ways.

Stratified drift deposits

The lowland valleys in Baxter State Park contain deposits of stratified drift which were formed by the melting of the final remnants of the ice sheet which last covered this area. Melting glaciers lose most of their volume by thinning, rather than by the retreat of their margins. During the wasting away of a continental glacier, the glacier eventually becomes so thin that hills and ridges protrude through the ice, leaving detached masses of ice in the valleys. The melting of these stagnant ice blocks produced the deposits of stratified drift which characterize the valleys in Baxter State Park, as in much of New England. Nearly all of the valuable sand and gravel deposits of New England consist of one kind of stratified drift or another.

Eskers

The most easily recognized of the stratified drift deposits are eskers, which are long, sinuous ridges of sand and gravel. These features are commonly called "horsebacks" or "whalebacks" in Maine.

Many geologists believe that eskers are formed in tunnels at the base of melting glaciers, tunnels bored by meltwater streams from the melting glacier. In time, deposits of sand and gravel from the melting ice build up in the tunnel, and when the ice finally disappears a ridge of this material is left upon the ground to mark the bed of the sub-glacial stream. Esker ridges range in width from about 20 feet to more than 600 feet, in height from 10 feet to more than 150 feet, and in length from less than 200 feet to more than 100 miles. Portions of some of the longest eskers in the world occur in the vicinity of Lincoln and Bangor, Maine.

Figure 14

Four principal groups of esker ridges occur in Baxter State Park and the nearby lowlands and are shown on the map of the glacial geology (Plate 1B). The most conspicuous esker occurs between Togue Pond and Abol Pond, a portion of which is shown in Figure 14. The subglacial stream which formed this esker apparently flowed toward the West Branch of the Penobscot River down what is now the Abol Stream valley.

Another esker system starts in the vicinity of Slaughter Pond, crosses the Nesowadnehunk Stream valley, and follows the lower part of the Katahdin Stream valley. It is characteristic of many esker systems that they show little regard for present drainage systems. Many occupy portions of several stream valleys. The esker system in the Wassataquoik Stream valley is one of those which diverges somewhat from the present drainage lines (see map of glacial geology, Plate 1B). In the valley between Billfish Mountain and Trout Brook and Horse Mountains, there is an esker which may be part of a long esker which follows the valley of the East Branch of the Penobscot River (see map of glacial geology, Plate 1B). This esker runs between High Pond (elevation 936 feet) and Long Pond (elevation 926 feet) and it is difficult to understand why the water does not seep through the esker so that both ponds have the same elevation. Perhaps this esker has some impervious material buried under the surface sand and gravel which prevents seepage.

Kames, kettles, and kame terraces

In addition to eskers, other types of stratified drift deposits occur in the valleys in Baxter State Park, but they are not as well developed and are generally difficult to recognize.

Small mounds of stratified drift are called kames and are commonly associated with eskers. It is believed that kames are formed by the settling of sand and gravel at the base of vertical pipes or wells in a melting glacier.

Kettles or kettleholes are depressions formed in deposits of stratified drift, caused by the melting of a large block of ice which had been buried in the sand and gravel. Many of the small, nearly circular lakes and ponds in the Katahdin region are kettleholes partly filled with water, as, for example, Russell Pond, Abol Pond, and Rat Pond (see Figure 14).

Kame terraces are accumulations of sand and gravel which occur at the sides of valleys and represent the deposits of meltwater streams that flowed along the margins of melting ice blocks. Terraces of this sort are present at Trout Brook Farm campsite in the northern part of Baxter State Park, and a gravel pit between the State Park road and the former Trout Brook Farm exposes typical stratified drift.

Drainage channels

Figure 15

Meltwater streams, besides forming the various kinds of stratified drift deposits described above, may also erode channels, which by their size and location may be distinguished from modern stream channels. In the valley of Trout Brook there is a complex system of meltwater drainage channels, most of which are now completely dry, although some are used by the modern streams. Because of the thick second growth forest which covers this part of Baxter State Park, it is difficult to see these features except from above, either from one of the nearby mountains, such as North Traveler, or from an airplane. Several of these channels are clearly visible at the upper left of Figure 15, a high level aerial photograph. Many channels, also visible on the Traveler Mountain quadrangle map, are shown as black arrow lines on the map of the glacial geology of Baxter State Park (Plate 1B).

Introduction Bedrock Glacial geology Geologic features Acknowledgments Glossary References Plates

Last updated on January 11, 2008

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