ME - From Fire to Foundation: Portland Geology EarthCache

The Geologic Origins of City Hall's Granite

Portland’s City Hall is a stately Beaux-Arts structure built in 1909, replacing an earlier building destroyed by fire. One of the most visually striking and geologically significant features is its exterior, constructed from North Jay Granite, a stone quarried in western Maine. This granite not only serves as a symbol of durability and civic pride but also provides an excellent window into the deep geologic history of New England.

What is Granite?

Granite is an intrusive igneous rock—meaning it formed from magma that cooled slowly beneath the Earth's surface. Because of this slow cooling, granite is composed of large, visible crystals of:

• Quartz (glassy, gray)
• Potassium feldspar (often pink or salmon)
• Plagioclase feldspar (white to gray)
• Biotite or hornblende (dark mica or amphibole minerals)

Granite is known for its strength, resistance to weathering, and aesthetic appeal—qualities that make it ideal for monumental architecture.

Geologic History of the North Jay Granite

The granite used in Portland City Hall was quarried from North Jay, Maine, and is part of the Newry Pluton, a large igneous intrusion that formed during the Acadian Orogeny about 370 million years ago in the Devonian Period.

What Caused the Granite to Form?

• During the Acadian Orogeny, a tectonic collision occurred between the Avalonian microcontinent and Laurentia (ancestral North America).
• This collision caused intense crustal thickening and subduction-related magmatism, where magma rose and intruded into the overlying crust.
• The magma cooled slowly and deeply, forming large granite bodies, or plutons, over millions of years.
• As erosion removed overlying rock, the granite became exposed at the surface and was later quarried in the 19th and 20th centuries.

Quarrying and Architecture

• The North Jay Granite Quarry opened in the 1880s and quickly became one of the most productive granite sources in the northeast.
• The stone is known for its pink to light gray color, coarse crystalline texture, and strength.
• In City Hall, the granite is used throughout the foundation, columns, steps, and facade trim, reflecting both durability and the Beaux-Arts design emphasis on grandeur and permanence.

References

Information above was referenced from the following sources:

• https://www.maine.gov/dacf/mgs/explore/minerals/quarries.htm
• https://www.maine.gov/dacf/mgs/pubs/map_series.htm
• https://www.portlandlandmarks.org/
• Winter, J. D. (2010). Principles of Igneous and Metamorphic Petrology (2nd ed.). Pearson Education.
• National Park Service. (2009). North Jay Granite Quarry District (NRHP Reference No. 09000938). U.S. Department of the Interior.
• Stone, W. E. (1920). Building Stones of the United States and Where to Find Them. U.S. Geological Survey Circulars.
• City of Portland Archives. (n.d.). Portland City Hall Architectural Files. Portland City Planning Department.

Activities

To log this EarthCache, visit Portland City Hall and answer the following questions based on your observations and information at the site. Please do not disturb or damage any part of the building

1. Texture Time! Examine the granite blocks in the exterior of City Hall. Would you describe the crystals as fine-grained, medium-grained, or coarse-grained? What does this indicate about the cooling rate of the magma that formed it?
2. Color Clues: What are the main mineral colors you see in the granite? (Look for pinks, whites, blacks, and glassy grays.) What minerals do you think these correspond to?
3. Weathering Watch: Can you find any signs of weathering or erosion on the granite blocks (such as pitting, staining, or crumbling edges)? If so, describe where and what kind.
4. Stone Story: Why do you think the architects chose granite for this building, based on what you now know about its geologic and physical properties?
5. Please take a photo of you or something with your caching name on it and post it in your log