Sunday Science - Sedimentary Rock

Being in St. Augustine for vacation offered a wonderful opportunity to look deeper into what makes a rock. The Fort Matanzas of St. Augustine is built out of Coquina, a unique sedimentary rock that is soft when quarried but hardens when exposed to air. The "curing" process takes 1-3 years. The very softness of the stone is what made its place in history. After twenty-seven days of bombardment by General James Oglethorpe in 1740, he gave up and left the city. The Fort was never taken by enemy fire. The reason the Coquina in St. Augustine's Fort was so effective against cannon fire is not its hardness but its softness. Cannon balls would sink into it rather than shatter or puncture the walls of the Fort. The Coquina itself turned out to be a terrific deterrent of hostile forces.Therefore the stone was frequently reused from older buildings in newer construction.

It is useful as an instructive tool because you can see the shells that go into the rock formation clearly even in quarried stone that has been cut and used in a building. Sedimentary rocks are made by compression of materials over time. Organic sedimentary rock is made from biological sediments which are frequently broken down and compressed to a point where you cannot identify the source. Yet in Coquina you the exactly what animal deposited the matter. We saw a living clam and then found that same shell in the stone. I explained time as glue. We stacked the shells and then investigated how hard it was to knock them over. We layered the shells with sand then experimented with knocking them over again. We stacked the shells one final time and pinned them down with a weight (the layers of soil) while she tried to knock them over. Then we quarried the stone (removed the weight) and watched how they tumbled. They needed the time to be the glue that turned it from our stack to the stone.

Our reading on the subject included The Magic School Bus Inside The Earth, and Let's Go Rock Collecting. Froggy did some rock collecting as we explored as well.


  1. What a fun idea.

    I went to St. Augustine (pre-kids) and loved visiting there. The fort is so cool!

  2. It sounds like a great place to visit. I like how you explained time working on the shells to "glue" them together.


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