The Geology of Baxter State Park and Mt. Katahdin

Geologic history of the Baxter State Park area

With the background provided by discussion of the bedrock types in Baxter State Park, it is now possible to outline the geologic history recorded in these rocks.

Relative age and absolute age of the rocks

Various kinds of evidence make it possible to determine the relative ages of two or more types of bedrock which occur in a given region. It has been mentioned previously that the conglomerate in the Trout Brook valley is younger than the Traveler rhyolite because the conglomerate rests on the rhyolite and because pebbles of rhyolite occur in the conglomerate. A third line of evidence by which relative age may be determined involves intrusive igneous rocks. A moment's reflection on the formation of granite will show that it (or any intrusive igneous rock) must be younger than the rock it intrudes, because there must be rock already formed into which or through which the granite can intrude.

But relative ages tell us only this rock was formed before that rock, and although such information is very important, we still want to know when. How unsatisfactory a history book would be if it told us only that Columbus discovered America after the Norman Conquest of England, but sometime before the Civil War!

Fortunately, some rocks contain certain minerals which record time as precisely as any calendar. Thus it is possible to determine with some accuracy how long ago the mineral and therefore the rock was formed. These minerals are termed radioactive minerals, the most familiar of which contain the highly active element uranium. Radioactive elements spontaneously change to some new element at a very definite rate of time, thus if the rate of time of this change is known, very accurate measurement of both the radioactive mineral remaining and the percentage of the new element present will indicate how long the radioactive change has been going on.

For example, starting with 1 gram of uranium, at the end of about 4.5 billion years (termed the half-life of uranium) there will remain ½ gram of uranium, the other ½ gram of uranium having changed to lead. Similarly, at the end of about 9.0 billion years, half of the remaining uranium will have turned to lead so that only ¼ gram of uranium is left.

By measurements of the amounts of uranium and lead and several other pairs of elements, the absolute ages of many rocks have been determined. Such techniques are most useful in dating igneous rocks.

Geologic time scale

The accumulation of evidence bearing on the relative ages of rocks from all over the world has resulted in a generally accepted measure of geologic time called the Geologic Time Scale. Geologic time is divided on geologic and fossil evidence into Eras, whose names characterize the stage of evolutionary development represented by fossils. "Paleozoic" Era means time of ancient life, "Mesozoic" Era means time of middle life and "Cenozoic" Era means time of recent life.

Eras are divided into Periods, the names of most of which refer to the geographic location of rocks first described as being formed during the period of the geologic time in question. For example, rocks formed during the Devonian Period were first described by geologists working in Devonshire, in southwest England. The name, Devonian, is applied to rocks formed during that period whether they occur in Texas, New York State, or Maine.

A simplified geologic time scale is shown in Table 1. Besides the subdivisions of geologic time, a few absolute dates are included, from which a perspective of the lengths of some of the Eras and Periods may be gained. Certain aspects of the fossil record of plant and animal life are also shown. It should be noted that nearly all of the rocks in Baxter State Park are assigned to the lower and middle part of the Devonian Period of the Paleozoic Era.

Older sedimentary rocks

The oldest rocks in the Mt. Katahdin area are the sedimentary rocks which form the bedrock in the lowlands surrounding Baxter State Park on the east, north, and west. These rocks are sandstone, shale, and limestone which range in age from Cambrian to Lower Devonian. The fossils which occur in these rocks are the principal evidence by which their age has been determined and because the majority of the fossils are remains of such marine organisms as brachiopods, corals, and molluscs, it must be assumed that these rocks were formed from marine sediments. For approximately 200 million years during early Paleozoic time the ocean covered nearly all of eastern North America, along with what is now northern Maine.

The distribution of these older sedimentary rocks is shown on the map of the bedrock geology, Plate 1A.

Traveler rhyolite

During the Lower Devonian Period, approximately 400 million years ago, volcanic islands were formed in an ancient sea by the accumulation of lava flows and beds of volcanic ash. The flows and ash beds form the Traveler rhyolite. Except for a brief time at the end of the Ordovician Period, this part of Maine was under the ocean during the early Paleozoic; however, the geologic evidence suggests that no important marine submergence occurred after the Lower Devonian Period.

Younger sedimentary rocks

The sedimentary rocks formed by the erosion of the volcanic islands are the conglomerates, sandstones, and shales which form the bedrock in much of the lower Trout Brook valley (see geologic map, Plate 1A). Thus, there are two main groups of sedimentary rocks in the Baxter State Park area: the older group, Lower Paleozoic in age, on which the Traveler rhyolite was deposited and the younger group, Lower Devonian in age, in part derived from the erosion of the Traveler rhyolite.

The formation of these sedimentary rocks, and their relation to the Traveler rhyolite, was discussed in detail above (see Figure 8).

An interesting feature of the younger sedimentary rocks is the presence of fossil land plants. Fossil evidence from many parts of the world indicates that few land plants, other than lichens, existed before the Devonian Period. Devonian age sedimentary rocks the world over contain abundant land plant fossils, as do all younger sedimentary rocks formed on or near land. Prior to the Devonian Period, the earth must have been a bleak place, indeed, with treeless, grassless, and flowerless landscapes. It is apparent from the fossil record that the earth became clothed in green for the first time in the Devonian Period. Undoubtedly it is no accident that the most primitive land-dwelling vertebrate animals (Amphibians) made their first appearance at this time.

Katahdin granite

The youngest rock in Baxter State Park is the Katahdin granite, which forms the bedrock in roughly the southern half of the State Park and in the lowland areas south of the State Park. At the power plant below Ripogenus Dam on the West Branch of the Penobscot River, there are several bedrock outcrops which indicate the Katahdin granite has intruded, and is therefore younger than the older sedimentary rocks described above. At the northwest end of Wassataquoik Lake the geologic relations indicate that the Katahdin granite has intruded the Traveler rhyolite also, and therefore is younger than the volcanic rocks in Baxter State Park. The somewhat more involved evidence which indicates that the Katahdin granite is also younger than the youngest sedimentary rocks, those exposed in the Trout Brook area, is discussed in the following section dealing with structural geology.

Radioactive age determinations of certain minerals in the Katahdin granite indicate it formed approximately 360 million years ago in the middle of the Devonian Period. Many other granites in New England were formed during the Late Devonian Period, although there is a wide range in age, from as great as 400 million years for some granites in Maine and Nova Scotia to about 350 million years for some Devonian granites in New Hampshire and Massachusetts.

Introduction Bedrock Glacial geology Geologic features Acknowledgments Glossary References Plates

Last updated on January 11, 2008

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