The Geology of Baxter State Park and Mt. Katahdin

Description and distribution of bedrock types

Katahdin granite

The bedrock in the southern half of Baxter State Park is granite (see geologic map, Plate 1A). Two distinct types of granite occur: the more common gray granite which forms the bedrock at intermediate and lower elevations and the pink colored granite which forms the bedrock at many higher elevations, especially near the summit of Baxter Peak. Both varieties are called Katahdin granite, from which it may be inferred that the rock in the Katahdin region is somehow different from any other granite. Trained geologists familiar with the Katahdin granite may quickly identify it, but to the untrained eye, Katahdin granite looks very much like many other granites.

The distinct difference in appearance of the two varieties of Katahdin granite is a result of a difference in the minerals which make up the granite. All granites are composed primarily of light colored minerals, feldspar and quartz, and contain minor amounts of such dark colored minerals as biotite (black mica) and hornblende. If the feldspar, which forms roughly 60 percent of the rock, is white or cream colored, the granite has an overall gray or white color, as in the case of the lowland variety of Katahdin granite. The presence of a pink or flesh colored feldspar produces a pink colored granite such as occurs at higher elevations on Mt. Katahdin.

This description of the Katahdin granite refers to its appearance in fresh, unweathered exposures. The granite in much of Baxter State Park, especially at high elevations, is covered with lichens which partly mask its true color. Many boulders of Katahdin granite on the Tableland and much of the bedrock along the Knife-Edge is colored greenish-gray, especially if viewed from a distance.

An interesting result of this lichen coloration may be seen on a sunny day along the trail from the Saddle to Baxter Peak. Loose boulders of both pink and gray Katahdin granite cover the slopes along this trail from the Baxter Peak Cut-Off Trail to the summit, but the rocks are colored greenish-gray by lichens. In the trail itself, however, the boulders have lichen-free surfaces because either the lichens are worn by traffic or are killed by the frequent changes in position of the boulders. When viewed from some distance, the trail is seen as a thin pink or gray strip in the general greenish-gray color of the boulder covered slopes.

Within the area of granite outcrops in the southern half of Baxter State Park, there are variations in appearance other than the general pink and gray color. Near Baxter Peak, the pink Katahdin granite has small holes lined with small crystals of a variety of minerals. The granite exposed near the summit of South Turner Mountain differs markedly in appearance from that exposed on Mt. Katahdin. This is because the individual mineral grains in the South Turner Mountain granite are much smaller than in the Mt. Katahdin rocks although the granites in both areas have about the same mineral composition. Because the mineral grains are smaller, the granite has a sugary appearance and it is difficult to distinguish individual minerals in outcrops of Katahdin granite on Turner Mountain.

In order to discuss the significance of the fine-grained granite of Turner Mountain, it is necessary to discuss briefly the origin of granites in general. Granite is one of the most common of the group which is called igneous rock, a term derived from the Latin, ignis, meaning fire. Many geologists believe igneous rocks have formed by the solidification of a molten or partially molten material called magma. The origin of magma is a widely disputed subject among geologists, but most of them agree on what happens to magma after it forms.

For example, it is generally accepted that volcanoes are formed by magma which has somehow reached the surface of the earth from its place of origin, probably several miles below the surface. Magma which has reached the surface is called lava. Molten lava cools, and in that sense, freezes to form various kinds of extrusive igneous rocks, meaning the rocks form at the surface of the earth, rather than below the surface. The bedrock in the vicinity of Traveler Mountain in the northern part of Baxter State Park is the result of the accumulation of various kinds of extrusive igneous rocks.

Much of the magma which starts toward the earth's surface fails to get there but cools and becomes solid rock at some depth below the surface. Igneous rocks which form in this fashion are called intrusive igneous rocks.

Experiments with artificially melted rocks and various theoretical considerations indicate that the rate at which magma cools will in part control the size of the individual mineral grains of the rocks which form. Other things being equal, the more rapid the cooling of a magma, the smaller the mineral grains of the rock will be.

Thus the mineral grains of extrusive rocks, lava flows and the like, which cool at the earth's surfaces, are often so small that they cannot be seen without magnification. Intrusive rocks, cooling below the surface, lose their heat very slowly, some taking thousands or even millions of years to become completely solid. The greater the depth, the longer the cooling period during which the individual minerals may grow in size. This simplified picture does not account for all the variations in grain size which occur in igneous rocks, but it may be taken as a general rule which can explain grain size differences in many geologic situations.

Those outcrops of Katahdin granite which contain large, easily recognizable mineral grains, probably cooled more slowly, at greater depths in the earth, than did the fine grained granite such as occurs on South Turner Mountain.

In nearly every outcrop of Katahdin granite there are cracks which are called joints. Some of the joints are the result of contraction of igneous rocks during cooling; others are caused by the extreme stresses produced by the rock surrounding the intrusion. In many outcrops there are numerous joints, some differently oriented, and as a result rock breaks from the outcrops in angular blocks with flat smooth surfaces. Katahdin granite exposed at Ledge Falls on Wassataquoik Stream breaks into extremely large blocks because the joints are widely spaced (Figure 1), whereas much smaller blocks of granite occur on the Tableland and the slopes leading from the Tableland to Baxter Peak (Figure 2). Well developed joints have produced large, nearly flat outcrops of granite along the Cathedral Trail and on the trail leading to the Pamola Caves. The Cathedrals themselves were formed by the erosion of granite which has prominent, closely spaced, vertical joints.

Figure 1

Figure 2

The presence of joints has weakened the granite to the extent that climbing over the granite is at best a nuisance and in many places on the Cathedral Trail, Saddle Trail, on the Knife-Edge and in the Chimney, care should be exercised in climbing lest a foothold give way. Well developed jointing also promotes more rapid weathering, which further weakens the granite.

Most of the trails on Mt. Katahdin cross many outcrops of granite and close examinations of the rocks along such trails as the Cathedral Trail or the Knife-Edge offer a convenient excuse for unscheduled rests. Remember, though, that there is not a great deal of variation in granite from one place to another, and too frequent "geologic" stops may cause the rest of the party to become suspicious of the motives for stopping.

Traveler rhyolite

North of the Katahdin granite area in the State Park, the bedrock consists of rocks of volcanic origin which may conveniently be called Traveler rhyolite. Rhyolite is an extrusive (see discussion of origin of granite) rock, and therefore composed of very small mineral grains. It has the same mineral and chemical composition as granite.

In appearance, Traveler rhyolite has a uniform dark gray or black groundmass which comprises the bulk of the rock, with a few larger light-colored mineral grains of quartz or feldspar. Thin, wavy bands of light-colored material occur throughout the rhyolite.

The weathered surfaces of outcrops of rhyolite may be nearly white, light gray, blue-white, or stained brown or red by iron, but freshly exposed surfaces are generally dark in color.

Outcrops of Traveler rhyolite occur along much of the two most commonly used trails in the Traveler Mountain area, the North Traveler Mountain Trail and the trail which leads along the shore of Lower South Branch Pond to the potholes on Howe Brook. The potholes on Howe Brook are formed in rhyolite. The geologic map, Plate 1A, shows the area of Baxter State Park in which the bedrock is Traveler rhyolite.

According to Douglas Rankin who has studied the volcanic rocks in the Traveler Mountain area in great detail, the Traveler rhyolite consists of many separate lava flows, one on top of another, which have a total thickness of several thousand feet. Rankin has found that besides the lava flows, deposits of volcanic ash, now firmly bonded into solid rock called tuff, also occur as part of the Traveler rhyolite. The hills and mountains in the Traveler Mountain area are not the peaks of volcanoes which once were active in this region, but probably the eroded roots or remnants of the volcanoes. Rankin estimates that some 80 cubic miles of rhyolite lavas formed in the Traveler Mountain area, making this one of the largest piles of this kind of volcanic rock in the world.

From a consideration of some rocks formed by the erosion of the Traveler rhyolite, Rankin visualizes the active volcanoes, if such these were, as a group of volcanic islands in a part of the ocean which covered this part of Maine more than 350 million years ago. More will be said of these islands and the rocks which were formed by their erosion in the section dealing with the geologic history of the State Park area.

Figure 3

One of the most interesting features of the Traveler rhyolite is the occurrence of columnar jointing in many outcrops. Columnar jointing, as the name suggests, causes the rock to break into long columns which usually have a more or less definite five or six-sided pattern, and is occasionally so perfectly developed that the columns seem to have been cut. Figure 3 shows columnar jointing on an unnamed hill in the headwaters of Gifford Brook. Jointing of this type forms as a result of contraction of lava during cooling.

Well known examples of columnar jointing in other parts of the world are the Giants Causeway in Northern Ireland and the Palisades along the Hudson River in New Jersey.

Unfortunately the blazed trails in the Traveler area do not cross outcrops which display well developed columnar jointing and the writer hesitates to suggest that anyone should leave the beaten path to search for geologic phenomena. The base of the first steep knob along the North Traveler Trail, about one-half mile from the South Branch Pond campground, is a vertical cliff formed by columnar joints, but these may be most easily seen from the south shore of the lower pond, or from a boat in the pond. Several easily accessible outcrops of columnar-jointed rhyolite occur on Dry Brook (Traveler Mountain quadrangle map, United States Geological Survey) less than a mile upstream from the Patten road. Columns in this area are smaller in diameter than those shown in Figure 3, but many are several feet long. Fragments of these columns have been carried down Dry Brook to its juncture with Trout Brook and careful examination of the gravel bed of Dry Brook should reveal some recognizable fragments of the columns.

Sedimentary rocks

As can be seen from the geologic map, the bedrock in most of Baxter State Park is igneous rock, either granite or rhyolite. In the northern part of the State Park and in the vicinity of Nesowadnehunk Lake the bedrock consists of various kinds of sedimentary rocks. Sedimentary rocks are formed by the accumulation, compaction, and cementation of sand, gravel, and mud. It will be recalled that a major difference between granite and rhyolite is the difference in the size of their mineral grains. In the same fashion, many sedimentary rocks are classified by the size of the rock and mineral fragments of which they are composed. A sedimentary rock composed of rock and mineral fragments about the size of sand grains is sandstone, whereas gravel-sized fragments form a rock called conglomerate. Shale is a sedimentary rock formed from mud, in which the rock and mineral fragments are much smaller than sand grains.

The origin of most sedimentary rocks is not as much of a problem as the origin of intrusive igneous rocks. It is possible to observe the present day accumulation and compaction of sedimentary rock-making materials, but it is doubtful that any geologist ever has or ever will be able to watch a granite form. One needs only to travel along the Maine coast to find bays and harbors in which mud and sand are accumulating. The mud in the clam flats will someday be converted to shale, and the long sandy beaches in southwestern Maine are potential sources of sandstone.

A common characteristic of sedimentary rocks is stratification, in which occur alternating layers of rock and mineral fragments of different sizes, commonly the result of variations in the strength of the waves and currents which transport the sediments. For example, a bed of mud may accumulate in the quiet recesses of a bay, to be covered by sand particles swept in during a storm. The rocks formed from these two sediments would consist of a layer of shale overlain by a layer of sandstone. Repetition of these conditions could form many horizontal layers. The illustrations shown below (Figure 5, Figure 6, and Figure 7) of sedimentary rocks exposed on South Branch Pond Brook show good examples of layering or stratification. Later, we will discuss the stratification found in certain glacial sediments.

Many sedimentary rocks contain features which suggest how the rock was formed. For example, ripple marks are preserved which do not differ in the least from ripple marks which may be seen on ocean and lake bottoms and in stream channels. The ripple marks indicate the size, strength and direction of the waves and currents which made them. By these and similar studies the geologist learns about conditions which prevailed when the rocks he is studying were formed. He is able to visualize the beaches, tidal flats, streams, oceans, lakes, swamps, and other features of some prehistoric landscape and thus reconstruct the long dead past.

Fossils

One of the most interesting and instructive features of sedimentary rocks are the fossils which occur in many of them. A fossil is any recognizable trace of life which existed before historic times. It may consist of a complete skeleton of an animal or a fragment of a tooth, a footprint or a worm hole, a perfectly preserved clam shell or only a faint impression of a shell, a petrified log or the carbonized impression of a leaf.

The geologist uses fossils in many ways to aid in the reconstruction of geologic history. For example, sharks now live in the ocean. Therefore, a fossil shark preserved in a rock would be good evidence that the rock was formed in the sea. Similarly, the fossil pine trees in the Petrified Forest, Arizona, indicate that the rocks in which the petrified wood is preserved were formed on land.

Many groups of animals and plants have not survived the changes of climate, the shifting of oceans and continents, and the periods of intense volcanic activity which have occurred in the geologic past. Their fossil remains are useful in dating sedimentary rocks, as well as suggesting the conditions that existed when the rocks were formed. For instance, it has been found from studies of rocks in many parts of the world that dinosaur fossils occur only in those rocks which were formed between roughly 225 and 70 million years ago. Thus if a geologist is studying a group of rocks of unknown age and finds some indication of dinosaurs, he knows those rocks are between 225 and 70 million years old.

The fossils which occur in the sedimentary rocks in Baxter State Park and vicinity may not be as spectacular as some famous dinosaur bone beds or the Petrified Forest, but they are nevertheless of great importance to the understanding of the geologic history of this part of Maine. There are few easily accessible outcrops of rocks containing fossils in Baxter State Park, yet fossils may be found in the gravels of beaches and streams in nearly every section of the State Park, especially in the vicinity of the South Branch Pond campsite.

The occurrence of these fragments of fossiliferous sedimentary rock in areas where the bedrock is granite or rhyolite is a result of the work of glaciers which covered this part of Maine, along with all of northeastern North America, several times during the past million years of Earth history. Glaciers ripped fragments from bedrock outcrops several miles northwest of where they are found today, carried them southward and redeposited them in areas where no fossils could possibly have originally occurred. This relocation of rocks and boulders illustrates why a geologist mapping bedrock geology must study only the real bedrock outcrops, rather than gravel and boulder occurrences, the source of which is uncertain.

Figure 4

The most common fossils in these glacially transported gravels are brachiopods, a type of marine animal which superficially resembles the clam, but is not even remotely related. Many rock fragments contain as many as ten different kinds of brachiopods, which are differentiated by the ridges and grooves on their shells, their general shape, their size, or some other feature of their shells. Figure 4 shows a rock containing various kinds of brachiopods.

Plant fossils occur in the shale exposed in downstream portions of Trout Brook. Some easily reached fossil occurrences are at Ripogenus Dam and along the north shore of Ripogenus Lake. Rocks in this area are sandstone and limestone and contain, in addition to brachiopods, fossil corals, bryozoa (a coral-like marine organism), and cephalopods (a marine mollusc, of which the chambered nautilus is the best known living representative.)

A recommended geologic field trip

Sedimentary rocks are dramatically exposed in the valley of South Branch Pond Brook between the pond and Trout Brook. The hike down this brook offers one of the most instructive and interesting geological excursions in Baxter State Park. But before attempting this trip one should consult the Ranger at the South Branch Pond campsite and examine the maps of the area, for there is no marked trail. It is recommended that South Branch Pond Brook be followed downstream to its juncture with Trout Brook and thence east along the south bank of Trout Brook to the Patten Road, a walk of slightly less than 3 miles. The trip from the campsite to the Patten Road will take about three hours.

The bedrock along the first mile of this trip is Traveler rhyolite (see geologic map, Plate 1A), and excellent exposures occur in several spectacular gorges, some of which cannot be passed without a refreshing swim. Several exposures have well developed columnar jointing, especially at a point about three-quarters of a mile downstream from the campsite.

Figure 5

Approximately one mile downstream from South Branch Pond the bedrock changes from Traveler rhyolite to conglomerate, a type of sedimentary rock described above. One of the best exposures is shown in Figure 5.

Careful examination of the geologic relations where the first exposure occurs will reveal the pebbles and cobbles in the conglomerate to be fragments of Traveler rhyolite, deposited upon bedrock of the same material. This very simple but important relationship deserves further mention, for it is a demonstration of one of the basic methods for interpreting geological phenomena.

It is clear that the conglomerate must have been formed after the lava flows of Traveler rhyolite were laid down; first, because the conglomerate is on top of the rhyolite and, further, because the gravel in the conglomerate contains pebbles eroded from the rhyolite. In other words, the conglomerate is younger than the rhyolite. Simply knowing the relative ages of two rocks, as in this case, can form the basis of understanding the geologic history of an area.

This example of relative rock age which is so clearly revealed in the rocks exposed along South Branch Pond Brook is, in the writer's view, the high point of the geology of Baxter State Park, worthy of mention with any exciting or dramatic geologic situation anywhere in the world.

Downstream from the initial outcrop of conglomerate the particle size of the sedimentary rocks gradually becomes smaller. Near the junction of South Branch Pond and Gifford Brook, the conglomerate contains small pebble-size particles and thin layers of sandstone (Figure 6). A few hundred yards farther downstream the exposed bedrock consists of even finer-grained sedimentary rock: sandstone and layers of shale (Figure 7). In the outcrop shown in Figure 7 and in several other outcrops downstream, there are thin layers of impure coal.

Figure 6

Figure 7

The decrease in particle size from conglomerate at the initial outcrop (Figure 5) to shale (Figure 7) is an interesting and important feature of the sedimentary rocks in the Trout Brook valley. In order that the geologic interpretation of these rocks may be presented as clearly as possible, a series of diagrams representing various stages in the formation of the rocks is shown in Figure 8.

Figure 8

Following the formation of the Traveler rhyolite lava flows and volcanoes, streams began to erode the rhyolite and in time an extensive gravel deposit accumulated along the lower slopes of the volcanoes (Figure 8A). Gradually the volcanic peaks became eroded to low hills and the streams attained gentle slopes over which only sand and fine gravel could be transported. Sand and gravel deposited in the stream channels covered the older, coarser, gravel deposit and formed a sandy plain (Figure 8B). During the final stages of sedimentation (Figure 8C) this became a low, swampy plain where accumulated mud and organic sediments formed shale and coal-bearing rocks. These now occur in the downstream portion of South Branch Pond Brook, while sand and gravel deposits have formed the sandstone and conglomerate which are exposed in the upstream portion. The older rocks occur upstream and are progressively younger downstream.

A moment's consideration of diagram C, Figure 8, will indicate that something has happened to the rocks between the stage represented by diagram C and the present. Sometime following the deposition of the sedimentary rocks they, with the underlying Traveler rhyolite, were tilted, and the various layers which were originally essentially horizontal (diagram C, Figure 8) became inclined downward toward the north. The stage shown in diagram D, Figure 8, occurred long after the tilting of the rocks, following a long period of stream erosion.

Also, the landscape represented in diagram D is not exactly that of the present, because various kinds of glacial erosion and deposition have modified the preglacial landscape. The geologic cross-section, Plate 1C, shows the geologic relations between the Traveler rhyolite and the sedimentary rocks in Trout Brook valley, and gives a more complete picture of the tilting, or folding of the rocks.

Introduction Bedrock Glacial geology Geologic features Acknowledgments Glossary References Plates

Last updated on January 11, 2008