Welcome to Acorn Street in Boston's historic Beacon Hill neighborhood! This street is a true hidden gem and is widely considered to be the most photographed street in America. Acorn Street has not been changed since the early 1800s and the cobblestone road still remains. This EarthCache focuses on these cobblestones used to build Acorn Street and their geologic makeup.
Please note: Acorn Street is a private way. While the public is permitted and Acorn Street is not private property, please be respectful of neighbors, as this is a residential street.
Acorn Street in 1890
Cobblestone
Cobblestone is a natural building material based on cobble-sized stones, and is used for pavement roads, streets, and buildings. It has been used across various cultures for millennia, particularly in Europe, and became especially prominent during the medieval and early modern periods. Today, cobblestone streets are often associated with historic preservation and are used in many cities to maintain the historical character of certain neighborhoods.
How did cobblestone get to Boston?
These naturally rounded cobbles, averaging about seven inches in length, may have come mostly from nearby beaches, such as Cohasset, Nahant, and Cape Ann. Later, however, they may have been carried by coastal schooners from the Maine coast in large quantities. To make a pavement, they were set one against the other, in beds of sand, their long dimensions vertical. From time to time, more sand was added to fill the voids between the cobbles. The stones may have also been quarried.
What is the cobblestone composed of on Acorn Street?
Paving records were sparse in the early 1800s, so it is not 100% definite what type of stone was used for the cobbles. Most educated guesses suggest that most cobbles were made of granite or Roxbury puddingstone, but gneiss, basalt, and schist were also likely used.
What is known is that the primary building material in Boston during the 19th century was granite. Besides being extremely strong and durable, granite was quarried in nearby Quincy, Milton, and Cape Ann (Gloucester and Rockport).
A town boundary marker between Milton and Quincy made of Quincy granite
The famed Roxbury puddingstone is the official stone rock of Massachusetts. Roxbury Puddingstone is a type of sedimentary rock which was formed millions of years ago when silt was compressed with smaller rocks and pebbles embedded within. Boston residents thought the pebbles in the pudding looked like fruit in pudding (like the Christmas pudding that is still traditionally served during the holidays in England). The nickname of Roxbury Puddingstone comes from a poem by Oliver Wendell Holmes about the children of giants who didn’t like their pudding and started a food fight. Since Roxbury Puddingstone is so plentiful in the area, early farmers had to dig it up in order to plant crops, so they used the stone for walls and foundations. This type of stone is known for having small pebbles that are visible.
Roxbury puddingstone in Milton
Some cobblestones here may also be made of gneiss. Gneiss is a coarse to medium grained banded metamorphic rock formed from igneous or sedimentary rocks during regional metamorphism. Rich in feldspars and quartz, gneisses also contain mica minerals and aluminous or ferromagnesian silicates.
A glacial pothole in gneiss in Shelburne Falls, MA
Basalt is a volcanic rock that is almost always dark in color. Basalt is relatively high in magnesium and iron content.
Schist is a metamorphic rock that shows visible flat grains that lie like sheets on top of each other. It is mostly made of long and flat minerals like mica, that sparkle in light. Schist is often finely mixed with quartz and other minerals. Schist appears to have layers and can be shiny due to high luster.
Does cobblestone erode?
In short, yes it can. Cobblestone may erode if it’s in an area with heavy water flow, melting ice, and subject to a lack of maintenance. Granite and basalt generally do not erode quickly, while gneiss and schist do.
Logging requirements:
Please message me with your answers in a timely manner. You may log your find before receiving verification.
1. What evidence did you find of either granite, puddingstone, gneiss, basalt, or schist in the cobblestones? Why would the builders of the street have used this stone?
2. Walk around Acorn Street and look closely at the stones. Do you notice any signs of erosion? What does this tell you about what type of stone makes up the cobbles?
3. Take a photo of you, your GPSr, or a personal item somewhere on Acorn Street.
Congrats to Lulu147 on the FTF!
References:
https://bostonsightseeing.us/acorn-street-boston#:~:text=Located%20in%20the%20charming%20Beacon,it's%20easy%20to%20see%20why.
https://www.archipedianewengland.org/1600-1699/historic-paving-and-sidewalks-in-new-england/
https://www.trolleytours.com/boston/acorn-street
https://nature.berkeley.edu/classes/eps2/wisc/gneiss.html#:~:text=Gneiss%20is%20a%20coarse%20to,and%20aluminous%20or%20ferromagnesian%20silicates.
https://www.britannica.com/science/basalt
https://rocksminerals.flexiblelearning.auckland.ac.nz/rocks/schist.html
https://www.usgs.gov/media/images/schist
https://en.wikipedia.org/wiki/Roxbury_Conglomerate