Geologic Walking Tour of Building Stones of Downtown Baltimore

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A rock is formed naturally on or inside the earth and is made up of one or more minerals. It is called a building stone once the rock is used in construction. Rocks are divided into three major groups—sedimentary, igneous and metamorphic—on the basis of how the rock formed. Sedimentary rocks are formed from sediments, such as gravel, sand, clay, or organic material, that have become cemented together naturally. Igneous rocks are formed from magma, or molten rock, as it cools and solidifies either deep inside the earth or on the surface when it is extruded as lava from volcanoes. Metamorphic rocks are formed when heat and/or pressure change or rearrange the composition of sedimentary, igneous or other metamorphic rocks. Metamorphic rocks often contain unique metamorphic minerals which help characterize these rocks.

A great variety and quality of native building stones used in Baltimore come from the geologic part of Maryland known as the Piedmont Plateau Province. Roughly the western half of the City of Baltimore lies in the Piedmont Plateau Province. Ranging in age from about 200 million to 1.1 billion years old, Piedmont rocks consist of granite, gneiss (pronounced “nice”), slate, marble, quartzite, and other rocks. Most of these rocks are igneous or metamorphic in origin, but a few rocks have a sedimentary source. The majority are hard, durable, attractive, and polish well, making good building stones.

Although some of the building stones observed on this tour were extracted from quarries in Maryland, most of the stones are not native to Maryland. An assortment of rock types was selected for this tour of downtown Baltimore, however, there are many more types of building stones located around the city which can be explored.

The map on this page shows the route of the walking tour. The route was selected to illustrate a variety of building stones in a small geographic area near downtown hotels. Some additional examples of the building stones that are described in this booklet, are present in buildings along the route between Stops, but are not specifically mentioned in the text for sake of brevity. After a while, the observer will be able to recognize some of the more common building stones (e.g., Indiana Limestone). The complete tour should take less than two hours.