Historical Bedrock Maps of Maine
Part II: The Keith (1933) Map

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, the 1967 Preliminary Geologic Map of Maine and the 1985 Bedrock Geologic Map of Maine. This fact sheet presents the second Maine bedrock map, by Arthur Keith in 1933. In comparing the maps, you will notice obvious differences in style and content. In each case we will 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.

	Preliminary Geologic Map of Maine 1933
	by Arthur Keith

Whole Map (large-size format) (4.9 Mb, pdf format)

Whole Map (medium-size format) (344 kb, gif format)

Map Legend (240 kb, gif)

Map Title (257 kb, gif)

History

The 1933 Maine bedrock map was a spin-off of the U. S. Geological Survey program to publish a geologic map of the United States. Arthur Keith of the U. S. Geological Survey compiled the information for New England at a scale of 1:2,500,000 and transferred it to the 1:1,000,000 scale you see here. An abstract was published in the Geological Society of America Bulletin in 1932. The Maine map was not widely circulated, having been published as a supplement to the Leavitt and Perkins (1935) volume on the Glacial Geology of Maine. Keith published papers on earthquakes, granite intrusion, and the regional bedrock structure of the New England crust but did not publish bedrock maps based on his own field work.

Geographic Index to the Map Figures

Lithology

Keith's map gives more emphasis to rock type than does Hitchcock's map, as reflected both by the Legend and by the portrayal of bedrock units. The two maps agree as to the essential character of Maine's geology, but the 1933 map shows a significant increase in complexity. New field study in the Rangeley-Jackman area greatly improved knowledge of that part of Maine (Figure K1A). Note also the convoluted contact between Silurian and Ordovician-Cambrian rocks in central Maine (Figure K1B). Excellent detailed studies published by the U. S. Geological Survey for Penobscot Bay (1907) and Eastport (1914) make those parts of the 1933 map comparable to the current (1985) map (Figure K2).

increased detail of greenstones near Rangeley and structure of units near Bingham

The most significant new information on Keith's map is the distribution of igneous rocks. Volcanic rocks - rhyolite, tuff, and greenstone - are shown to comprise a northern Maine belt from Rangeley to Presque Isle and a coastal belt from Penobscot Bay to Eastport (Figure K3). Even though they occur mostly in small patches, their importance is emphasized by the bright colors on the map. Intrusive rocks are shown carefully as well. The many small plutons south of Moosehead Lake (Figure K4) can be readily recognized on the current state map (Onawa, Hartland, Old Point, e.g.). Curiously, the Androscoggin Lake pluton, which was shown by Hitchcock, is omitted from the 1933 map.

Age

About the only new age information since the 1860's came from new fossil finds in Penobscot Bay and Eastport which corroborated earlier information. The age assignments of Hitchcock are adopted, with a few notable exceptions. A patch of rock in southern Maine is assigned to the Pennsylvanian by erroneous correlation with fossiliferous Pennsylvanian rocks in Massachusetts. Rocks near Presque Isle and Perry are shown as Mississippian, although subsequent work has proved them to be Devonian as Hitchcock had shown. The conventional assignment of New England metamorphic rocks to the Precambrian is followed here for the last time, as a 1934 paper by M. P. Billings and A. B. Cleaves was about to show that fossils near Littleton, New Hampshire, (originally discovered by C. H. Hitchcock in 1870) indicate that metamorphic rocks in central New Hampshire are Paleozoic.

Ages of the igneous rocks are deduced from their relationships to the sedimentary rocks. The Red Beach Granite in eastern Maine is assigned to the Devonian, because it cuts across Silurian strata and is overlain by "Mississippian" strata. Granites of southernmost Maine that intrude "Pennsylvanian" strata are interpreted to be Carboniferous. Granites, pegmatites, and migmatites of western Maine are purported to be Precambrian because of their intimate relationship with the metamorphic schists and gneisses assigned to the Precambrian. Although these relative geologic relationships are reasonably deduced, they gave incorrect results in many cases because the presumed ages of the sedimentary rocks were not correct.

Regional Relationships

According to the Legend, the greenstones are mostly Silurian and the rhyolites are Devonian. This implies a change in the nature of volcanism over time. It also indicates that there are regional variations in the types of rock being formed at any one time. During the Silurian, for example, sandstone and shale were being deposited over much of Maine, whereas volcanic rocks were being erupted in certain areas of the Down East coast and northern Maine. This gives a more dynamic and varied picture of Maine's geologic history than Hitchcock's map implies. The upper contact of the Precambrian mimics the upper contact of the Cambrian-Ordovician unit near Farmington, suggesting that a regional folding event affected all the rocks together (Figure K5). A profound unconformity above the Precambrian rocks is prominently displayed.

map showing implied structural relationships

Figure K5

Conclusions

The differences between the bedrock maps of Maine derive partly from the amount of information available at the time of publication, but depend more importantly on the perspective of the author. Keith's map includes more detail than Hitchcock's, and a more ambiguous geologic history. Each map emphasizes a different aspect of Maine's bedrock geology, and both maps had their place in the progress of scientific thought.

References

Bastin, Edson S., and Williams, Henry S., 1914, Description of the Eastport quadrangle, Maine: U. S. Geological Survey, Geologic Atlas of the United States, Folio 192, 15 p.

Billings, Marland P., and Cleaves, A. B., 1934, Paleontology of the Littleton area, New Hampshire: American Journal of Science, 5th series, v. 28, no. 168, p. 412-438.

Keith, Arthur, 1923, Outlines of Appalachian structure: Geological Society of America, Bulletin, v. 34, p. 309-380.

Keith, Arthur, 1932, New geologic map of Maine (abstract): Geological Society of America, Bulletin, v. 43, no. 1, p. 171-172.

Keith, Arthur, 1933, Preliminary geologic map of Maine: Maine Geological Survey, issued as Supplement to Leavitt, H. W., and Perkins, E. H., 1935, A survey of road materials and glacial geology of Maine, Volume II - Glacial geology of Maine: Maine Technology Experiment Station, Bulletin 30, vol. 2 (scale 1:1,000,000).

Keith, Arthur, 1933, Orogeny of Maine granites (abstract): Geological Society of America, Bulletin, v. 44, pt. 1, p. 89.

Smith, George Otis, Bastin, Edson S., and Brown, Charles W., 1907, Description of the Penobscot Bay quadrangle, Maine: U. S. Geological Survey, Geologic Atlas, Folio 149, 14 p. (map scale 1:125,000).

Text by Henry N. Berry IV

Graphics by Marc Loiselle and Henry Berry

Originally published on the web as the July 2004 Site of the Month.

Last updated on April 11, 2012

Historical Bedrock Maps of Maine Part II: The Keith (1933) Map