Historical Bedrock Maps of Maine
Part III: The 1967 Preliminary Geologic Map of Maine

Introduction

Since William Smith presented the first modern geologic map in 1815, of England and Wales with part of Scotland, geologic maps have become the standard way of portraying geologic information. There have been four significant maps showing Maine's bedrock geology, published in 1885, 1933, 1967, and 1985. Each map is a product of its time, reflecting the current status of knowledge and understanding of geologic principles by the author. So it is best to view each one as a progress report, representing an interpretation of myriad bits of information.

In using a geologic map, or any map for that matter, it is important for the reader to appreciate the purpose for which the map was made. Certain things are shown and many things are not shown. Coloration and labeling emphasize things of perceived importance. In preparing a geologic map, hundreds of decisions are made by the geologist, and scrutiny of the finished map gives insight into that thought process.

In other fact sheets we look at Charles Hitchcock's 1885 map, Arthur Keith's 1933 map, and the 1985 Bedrock Geologic Map of Maine. This fact sheet presents the third Maine bedrock map, The 1967 Preliminary Geologic Map of Maine. In comparing the maps, you will notice obvious differences in style and content. In each case we review the context in which the map was made (History), point out the way in which different rocks are portrayed (Lithology), discuss the geologic ages assigned to the various rocks of Maine (Age), and look at major geological relationships that can be deduced from reading the map (Regional Relationships). Selected close-up areas are presented as figures to illustrate these points.

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1967 Preliminary Geologic Map of Maine

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Whole Map (381 kb, gif format)

History

Figure 1

The 1967 Preliminary Geologic Map of Maine was the first statewide effort that drew on a multitude of compilers, editors and advisors (Figure 1), in contrast to the two earlier bedrock maps which were single-authored compilations. The State Geologist at the time, Bob Doyle, provided this historical perspective:

Geographic Index to the Figures.

Lithology

The impact of detailed mapping on the 1967 state map is dramatic. In every place where new work was done, stratified rocks shown as a single unit on the 1933 map are now shown to be lithologically complex. Whereas the earlier state maps grouped similar rocks together to gain a general sense of regional geology, this map portrays local variability. Individual rock units, some very thin, are traced for great distances, emphasized by coloration and patterning (Figure 2) on the map. Perhaps the most significant advance is the separation of the intensity of metamorphism, shown in a metamorphic inset map (Figure 3) at the upper left, from the primary stratigraphic aspect of rock units. Notice that some individual units are mapped continuously from low metamorphic grade to high metamorphic grade; this is not the case on earlier maps. Stratigraphic relationships can be considered independently from the younger metamorphic overprint.

Figure 2

Figure 3

Intricate stratigraphic relationships are shown, including facies relationships and lenticular members within formations. Volcanic rocks, while confined generally to the northern and coastal belts shown on the 1933 map, are complexly interstratified with sedimentary rocks. The Explanation on the right half of the sheet emphasizes the rock types (Figure 4) of each unit, and gives less importance to formation names (listed in parentheses). The patchwork nature of the progress map is evident where thin units end abruptly against units of "undifferentiated" rock. There is even a blank swath with question marks (Figure 5) through east-central Maine, testament to the incomplete state of knowledge.

Figure 4

Figure 5

Areas of dramatic progress since 1933 include the plutonic belt along the Down East coast (Figure 6), stratified rocks of eastern Aroostook County, Devonian and Ordovician volcanics around Chamberlain Lake (Figure 7) (northern Maine), Devonian strata of the Moose River area (Figure 8) (west of Moosehead Lake), the metamorphic region from Mt. Katahdin southwest to New Hampshire (Figure 9), and the structurally convoluted Waterville rocks (Figure 10). These parts of the map did not change significantly for the 1985 map, although the connections between them are less abrupt on the later map.

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Other notable details shown on the map include patterned areas affected by contact metamorphism around plutons (Figure 11), thin red lines representing Mesozoic mafic dikes (Figure 12), and heavy black lines indicating faults (Figure 13).

Figure 11

Figure 12

Figure 13

Age

Based on much new paleontology and a few crude isotopic age dates, all the stratified rocks are assigned to the Lower Paleozoic, including the metamorphic rocks of western Maine which have settled into their proper place in the Silurian Period. No Precambrian rocks are shown. A few plutonic rock units in southwestern Maine are assigned to the Mesozoic. Age is indicated by capital letters which begin each unit symbol. Uncertainty is illustrated on the Explanation (6.9 Mb, pdf format) by brackets showing lower and upper age limits. In some cases the brackets are quite large, giving the reader an appreciation of the variable quality of age data and stratigraphic control in different parts of Maine.

Regional Relationships

Appropriate to its name, this "Preliminary" map is largely descriptive, showing many details, but without presenting an integrated regional concept. Nonetheless, several fundamental relationships are shown. Northern Maine is dominated by Devonian strata (purples and blues) with inliers of pre-Devonian rock. A large Silurian basin (greens) extends from northeastern Maine through central Maine into New Hampshire. Some patches of older rock appear near Danforth in eastern Maine (pink) and along the coast eastward from Penobscot Bay (pink, yellow, gray). The coastal belt includes volcanic rocks intruded by abundant granite and gabbro plutons (patterned). The rocks of central and western Maine are intensely folded, much more severely than previous maps indicate. Devonian plutons cut across map-scale folds, dating the major deformation to Devonian. In western Maine, stratified rock units are remarkably continuous and traceable through areas of complex deformation, plutonism, and high metamorphic grade.

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Text by Henry N. Berry IV.

Graphics by Marc C. Loiselle

Originally published on the web as the August 2007 Site of the Month.

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Last updated on March 7, 2008

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