Geologic Maps

by Joe Kopera

All geologists, when you get down to it, are trying to figure out how our planet works. Given that the Earth is essentially a large (albeit very active) rock, it’s no wonder that rocks tend to be the focus of our attention. Geology has been described as detective work:  the deductive reasoning commonly applied by geologists trying to glean the story from a given rock, mineral, or fossil is similar to the sage cogitations of Sherlock Holmes (arguably the first forensic geologist to appear in literature, by the way, in A Study in Scarlet). Thus, when trying to deduce the what? and how? of a rock, mineral, or fossil at the Jurassic Roadshow this Saturday, the geologists there will look for potentially dozens of clues, including the minerals present, mineral grain size, color, shape of the fossil, and other characteristics.

Geologic maps show the geographic distribution of rock strata, or formations, in a given area. They are still the most fundamental way to communicate what rocks underlie an area and how they are situated in relation to one another. Until recent years, a degree requirement for almost every geologist was to spend a summer at a geologic field camp learning how to make these maps, using detective skills to assemble and analyze all the clues about a particular landscape. Through the process of making a map, a geologist can get a pretty good idea of the geologic history of an area and then communicate that knowledge to others through the map.

A clue of equal importance is where? Knowing where a rock comes from gives geologists information crucial to deciphering the larger story. Thus, the modern study of geology is largely about context. Rocks, strata, minerals, and fossils do not occur randomly, but in a system. Over the past two centuries, the study of how various types are distributed with respect to one another, and their occurrence around the globe, has taught geologists the fundamentals of how rocks are formed, and this knowledge gave geologists tremendous insight into how the planet works. One of the most recent, and perhaps most paradigm-shifting, observations led to the model of plate tectonics in the 1970s. Study and learning continue to this day through real-time satellite-based observations of the plates actually moving. Astounding!

In 18th and 19th century England, when the modern science of geology was in its infancy, coal miners noticed that certain fossils were associated with certain rock strata in mine shafts. They also noted that such strata occurred in a certain order relative to one another. William Smith, an English canal engineer who also worked in the coal mines at the turn of the 18th century, took note of the distribution of fossils and strata all over England, and began to realize that the fossils he saw always occurred in a specific order. He realized (much to his profit as a mining prospector) that he could determine where he was in the order of the strata based on the fossil assemblages present in a given rock outcrop or mine shaft, and that he could trace certain strata across the landscape of England. Smith published what is thought to be the world’s first national-scale geologic map in 1815, the account of which is told in Simon Winchester’s book The Map That Changed the World.

Time spent with a well-made geologic map can tell you a lot of things about an area aside from just where certain types of rock are located. The maps show what rock and sediment types are under which landforms, which explains why a landscape looks the way it does. Maps can help geologists to forecast where natural hazards may be more prone to occur: steep slopes in combination with the right (or wrong) soil type, or certain orientations of features such as layering or fracture systems, can indicate that an area is likely to have landslides, for example. A geologic map portraying certain rock formations containing high proportions of elements such as arsenic and uranium can help public health officials create guidelines for water well drilling. Geologic maps are used extensively to help municipalities and states manage and locate sources of groundwater, which is the sole source of clean drinking water in many parts of New England. Such maps are also key tools in properly siting and designing geothermal and ground-source heat pump systems, increasingly popular for heating buildings in New England. They are also used in large construction projects to predict subsurface conditions that can result in expensive contractor claims. Such uses are among those that demonstrate 27-to-1 return on investment in geologic mapping, according to recent studies (

While a state-wide geologic map of Massachusetts exists, the Commonwealth is far from being completely mapped at a level of detail that is useful for local purposes. It is comparable to having only a state highway map at your disposal if you want to navigate Greenfield. The Massachusetts Geological Survey, in partnership with the United States Geological Survey (USGS) and local colleges and universities, is working to create complete coverage of the state. We still have a long way to go, as it takes 3-5 years for a geologist to produce a detailed geologic map of a 7.5′ topographic quadrangle (55 square miles), but  we are making steady progress. Come take a look at our work at Jurassic Roadshow, under the tent on Main Street near Federal Street.

If you are interested in looking at more geological maps, take a look at the National Geologic Map Database:

One thought on “Geologic Maps

  1. Hello, I think your website might be having browser compatibility issues.
    When I look at your blog site in Opera, it looks fine but when opening in Internet Explorer, it has some overlapping.
    I just wanted to give you a quick heads up! Other then that, wonderful blog!

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