Bedrock Geology - Frequently Asked Questions


Q1. What is bedrock?     Back

Bedrock is the solid rock that makes up the earth's crust. It forms a continuous foundation beneath the whole state, but in most of Maine, bedrock is covered by some thickness of surficial sediments, soil, and vegetation. Bedrock, or ledge, is commonly encountered in shallow excavation projects, such as digging for foundations or blasting for road construction. In addition, there are widespread natural exposures of bedrock on hilltops, where overlying materials have not accumulated, or along streambeds and shorelines where bedrock has been washed clean. Maine has an abundance of boulders and stones in the glacial deposits that do not qualify as bedrock since they are no longer solidly attached below ground, but they can be just as troublesome for drilling and excavation projects.


Q2. I want to drill a well or do excavation work. How far down is it to bedrock?     Back

Compared to many parts of the world, surficial materials in Maine are relatively thin, generally less than 50 feet, and rarely over 100 feet. That having been said, the bedrock surface in Maine is quite irregular, and difficult to predict in detail. If it is necessary to know the depth to bedrock precisely, specific work at the site is required, such as test borings or geophysical surveys (by seismic refraction, or ground-penetrating radar). This is done for bridges and major engineering projects. For less critical situations, the MGS produces four series of maps that include data on bedrock depth for specific points, and these may be helpful depending on the complexity of the local geology:

  • Surficial geologic maps show areas of "thin drift", by an overprint pattern. In these areas, the geologist interprets bedrock to be 10 feet or less below the surface.
  • Surficial materials maps show measured thicknesses of surficial materials and depth to bedrock from well logs and test borings at specific places where information is available.
  • Detailed bedrock geologic maps show locations of bedrock outcrops, which are places where the bedrock is at the ground surface.
  • Bedrock groundwater resource maps show overburden thickness as estimated by water well drillers. This information is presented in groupings of 0-5 ft, 5-10 ft, 10-15 ft, 15-25 ft, etc.

These maps are helpful in giving an idea of what is typical for an area, but they do not make exact predictions for specific places.


Q3. What kinds of rocks make up the bedrock of Maine?     Back

Taken as a whole, Maine's bedrock comprises a vast array of rock types, some common and some rare, each with variations in mineral content, color, texture, and structure. Geologists classify rocks and assign them names based on certain basic characteristics, but the degree of natural variation is almost limitless. All three major rock groups - sedimentary, igneous, and metamorphic - are represented in Maine.

Here is an abbreviated list of Maine rock types. Sedimentary rocks: shale, mudstone, siltstone, various sandstones, arkose, graywacke, chert, limestone, conglomerate. Igneous rocks (volcanic): basalt, andesite, dacite, rhyolite, various tuffs and breccias. Igneous rocks (plutonic): granite, pegmatite, quartz monzonite, syenite, diorite, diabase, gabbro. Metamorphic rocks: slate, phyllite, schist, granofels, various gneisses, marble, quartzite, greenstone, amphibolite, serpentinite, calc-silicate rocks, hornfels, migmatite, mylonite.

For the distribution of rock types across Maine, refer to bedrock geologic maps. For photos and descriptions of bedrock sites, take a virtual tour (pdf format - 1.8 Mb), or visit our field localities pages.


Q4. Where in Maine can I find granite (or limestone, or some other particular kind of bedrock)?     Back

Bedrock geologic maps show where different kinds of bedrock are found. The general statewide distribution of rock types is shown on the Bedrock Geologic Map of Maine. It can guide you to areas of the state that have the type of bedrock you are interested in. Many rock bodies in Maine are very small or have irregular shapes, so more detailed bedrock maps will be needed to find locations on the ground. References to many detailed maps are given on the sidebar of the Bedrock Geologic Map of Maine itself. Additional maps and reports may be found through a KEYWORD SEARCH of our publication search page.

If you are looking for commercial sources of Maine stone or aggregate, there are many local suppliers of construction aggregate, building stone, or landscaping stone across the state. Look in the phone book or search the internet, keeping in mind that in the global market local suppliers provide stone that comes from outside Maine.

The USGS compiles annual mineral industry reports that estimate the volume and value of mineral resources produced in Maine. A list of active licensed quarries is maintained by the Mining Coordinator of the Maine Department of Environmental Protection. This list significantly under-represents existing Maine stone resources, since small or inactive quarries are not required to be licensed. In addition, a certain amount of stockpiled Maine stone may be available from quarries or dealers that are not actively quarrying.


Q5. How can I get a bedrock map for my town?     Back

Geologic maps are normally published by rectangular areas called "quadrangles" (defined by a uniform latitude-longitude grid), not by town. In a few cases, a mapping project may concentrate on a particular geologic feature, producing a map of an irregular geographic area. To find available maps that cover your town, go to the MGS publication search page, and do a GEOGRAPHIC SEARCH by Town. Be aware that the available level of detail is not uniform across the State, so some areas may be covered only by the generalized State map.


Q6. Is the rock at Two Lights State Park petrified wood?     Back

Although some pieces have a strong resemblance to old weatherbeaten wood, the rocks along the southeastern shore of Cape Elizabeth and Scarboro are metamorphic rocks derived from sandstones. The sheet-like structure that looks like wood grain is a metamorphic foliation produced by aligned silicate minerals that grew under pressure at depth in the earth. This internal metamorphic structure causes the rocks to break into jagged pieces like a splintered log. But their derivation from sedimentary rock is clearly demonstrated by the sedimentary layers that can be seen in many places. (For more, see our Field Locality page for Two Lights State Park (PDF, 1.3Mb).)


Q7. Is Mount Katahdin a volcano?     Back

There is a grain of truth to this myth, in that there are ancient volcanic rocks preserved nearby, in northern Baxter State Park, but none of the mountains of Maine's modern landscape are actually volcanos. Volcanos are landforms that were constructed by volcanic processes. All Maine's mountains are erosional remnants. The rocks of Mt. Katahdin, in fact, are not volcanic rocks at all, but are granite, an igneous rock that forms by slow cooling of molten rock beneath the earth's surface. It cannot be denied that the view of Mt. Katahdin from the northeast, with its bowl-shaped cirques carved by glaciers, superficially resembles that of a volcanic complex with summit calderas (compare with Mount St. Helens). The low-lying surrounding topography, underlain by more easily eroded sedimentary rocks, enhances the stand-alone effect.


Q8. How did Maine's mountains form?     Back

Maine's present landscape has formed primarily by erosion. Even the tops of mountains have bedrock that formed at depth in the earth and has been uplifted and eroded. Our mountains are simply places where the bedrock has been worn down less than the bedrock of the neighboring areas. Different rocks resist erosion to different degrees, depending on their composition, texture, and structure. This phenomenon, termed differential erosion, accounts for the main landscape features of geologically old mountain belts such as the Appalachians.


Q9. Are there caves in Maine?     Back

Yes. Many small caves of various types have been discovered in Maine. Some are below ground, but many are openings on steep slopes or cliffs. The local chapter of the National Speleological Society, the Boston Grotto, provides information and training about "caving." One of their members, Eric Hendrickson, has created a comprehensive web page about Maine caves. Anyone interested in visiting caves should be familiar with the Maine Cave Protection Act, which respects private landowner rights as well as the natural, historical, and archaeological value of caves.


Q10. How old is the bedrock of Maine?     Back

The oldest reliable age determined for a Maine rock is 647 million years old (with analytical uncertainty of 3.7 million years), for a pegmatite near Islesboro in Penobscot Bay. Interestingly, this pegmatite cuts through rocks of the Seven Hundred Acre Island Formation, so we know that that formation is older than the pegmatite, but we don't know by how much.

Individual mineral grains from the Chain Lakes massif, north of Eustis, have yielded a wide spectrum of ages, the oldest dated at about 2800 million years old (that's 2.8 billion). These old grains are interpreted as a variety of sedimentary grains shed from an older continent and incorporated into a younger sedimentary rock. The rock itself is about 485 million years old. So while this rock is not as old as the pegmatite in Penobscot Bay, it contains within it a few tiny mineral grains that are very old indeed.

The vast majority of Maine rocks are of Early Paleozoic age, from about 360 to about 510 million years old, representing 150 million years of earth history. Episodes of sporadic geologic activity occurred at younger times, producing relatively small amounts of igneous rock in restricted areas at around 290-300 million years ago, 200 million years ago, and 120 million years ago. The youngest bedrock known in Maine so far is a small igneous body in the town of Parsonsfield, the Randall Mountain stock, which has been dated at 104 million years old (with 4 million year uncertainty).


Last updated on March 29, 2013