Baltimore Rocks

Baltimore Rocks

The wide variety of stone types used to face and decorate the buildings of downtown Baltimore gives the careful observer a geologic history of the world.  

“This is probably the oldest rock in Baltimore,” Sam Glasscock said, gesturing toward the polished, gray and pink Morton gneiss that clads the front of the downtown Police Headquarters on East Fayette Street. 

“It’s about 3.6 billion years old,” he added. 

Glasscock, an ocean geologist and environmental scientist, was our guide for the Natural History Society of Maryland’s “Geology Tour of Baltimore’s Buildings and Monuments.” It turned out to be a fascinating expedition. 

 He pointed out that the oldest rock on the surface of the Earth, bedrock emerging from beneath the tundra on the north shore of Hudson Bay, is 4.3 billion years old. (Erg Check 02, a meteorite discovered a few years ago in the Sahara of Algeria, is believed to be 4.6 billion years old, older than the Earth.)  

To put that in perspective, the oldest stone native to Maryland is “only” 1.1 billion year old. Coincidentally, most of that is a kind of rock known as Baltimore Gneiss. Domes of this basement rock rise close to the surface in several parts of the Maryland Piedmont. Crags of the primordial stone are visible where the younger rock above it has eroded away. One of the best places to see this hard, green streaked stone is along the banks of Catoctin Creek, just south of Middletown on Rte. 17. 

The greenish color of Baltimore Gneiss is principally due to the presence of amphibole, one of its signature minerals. Glasscock explained that Morton Gneiss, which is quarried in Morton, Minnesota, gets its color from contorted layers of black mica and pink feldspar interspersed with gray quartz. These gaudy bands give the stone its common name: Rainbow gneiss. 

To put all this into perspective, Glasscock began the tour with a short refresher course on the three basic types of rock. 


Sedimentary rock, he pointed out, is comprised of tiny mineral particles that have been compacted or cemented together over time. Limestone, for example, often consists of the tiny shells of billions marine organisms that, over the millennia, settled on the floors of ancient seas. Calcium in the seawater cements the particles together into thick bands of homogeneous stone. 

Travertine is another kind of limestone created when calcium and other minerals precipitate out of mineral springs. The stalagtites and stalagmites in caves are examples of travertine. Although typically tan or gray, the color can vary greatly due to the type and quantity of trace elements in the stone. 

Sandstone and shale are also sedimentary rocks — in this case, formed from sand or miniscule grains of mud. 

Igneous rock is formed when molten magma solidifies. There are two types: Extrusive rock forms when lava flows onto the Earth’s surface. If it cools slowly, larger crystals can form to create basalt or pumice. If it cools quickly, Glasscock explained, the crystals remain small or absent altogether, as in the case of obsidian glass — which, like all glass, is technically a fluid. 

Intrusive rock forms when magma cools beneath the Earth’s surface. If it cools quickly, such as when magma seeps into a fissure between two cool masses of stone, it can form dikes or sills of hard, fine-grained stone such as the big slabs of diabase that form Stonehenge. 

If it cools more slowly, larger crystals have time to form, creating common stones, like granite or gabbro. 

The third category of rock is metamorphic, which is formed when sedimentary or igneous rock is subjected to enormous pressure and heat. Millions of years under the weight of a mountain range can convert sedimentary limestone into marble, igneous granite into gneiss, shale into slate. 

All this information became more meaningful as Glasscock led our little group of explorers through the streets of downtown Baltimore. He showed us examples of all the major types of stone. Warned ahead of time, most of us had a magnifying glass or hand-lens to get a better look at the geode-like crystals of Finnish granite or the multitude of tiny fossils that comprise Indiana limestone.  

Not all fossils require a magnifying glass, though. At the corners of the War Memborial Building on N. Gay Street, Glasscock took us to see a pair of massive horse statues. The limestone plinth beneath one of those statues is criss-crossed with clearly visible trace fossils of worm-like animals that once lived in the muck at the bottom of an intra-continental sea. 

A Geological Tour of the World 

That’s one of the main charms of the Natural History Society of Maryland’s urban expedition. Because of the international nature of the market for fine construction material, a five or six-block loop through downtown Baltimore is almost a short course in the geologic history of the world. At the very least, it will give you a closeup peak at some of the world’s most beautiful and fascinating stone. 

Along the way, you’ll see local stone products. Polished Cockeysville marble from the old Beaver Dam Quarry north of Baltimore dresses the base of the Washinton Monuments in both Baltimore and Washington, D.C. Similarly, the intricately carved Seneca sandstone facade of the old Merchantile Trust and Deposit building was also used to build the Castle at the Smithonian Institution. Glasscock will also take you to see stone from farther afield: that Indiana limestone, for example, or gneiss from Georgia. 

Then there are the beautiful exotics: slabs of large crystal red granite imported from Finland; great slices of Spanish travertine that, at first glance, looks like enormous panels of wood; and Italian marble from quarries that have been in operation since the Renaissance. 

Almost the entire history of the Earth is written in the dimension stone adorning the buildings of Baltimore. But the tour is also a reminder of the human enterprise and vast infrastructure that goes into the use of stone in architecture.  

Consider that Italian marble. The famous Carrera marble that clads the facades so many buildings and monuments around the world was typically quarried in massive slabs — as big as 15 feet long, eight feet wide, and four feet tall. Those slabs might have been transferred from the hills of Tuscany to a processor in Germany where they were sawed into three-inch slices. Another plant probably cut the slices into the precise dimesions required by the builder. Still others polished or flame-treated or chiseled the surface to give it the appropriate finish. The face might also have been coated with epoxy or resin to fill any voids before being polished.  

Then the marble slabs had to be individually crated, numbered, and shipped from the European finishing plant to Baltimore, where stone specialists prepped them and eventually attached them to the sides of the city’s skyscrapers or installed them at the bases of the city’s monuments. 

This is the sweeping story the soft-spoken Glasscock tells during “Geology Tour of Baltimore’s Buildings and Monuments.” It will forever change how you look at the architecture of Charm City — and cities everywhere. 

Dennis Hollier

Dennis Hollier

Dennis is a travel, science, and business writer who has traveled all his life. The son of an Air Force pilot, he was born in England and lived in ten states growing up. Much of his youth was spent in Hawaii and Southeast Asia, where he traveled widely, including extended visits to New Delhi, Singapore, Hong Kong, Vientiane, and old Rangoon.

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