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Calthemites at Accotink Creek Bridge

A cache by hzoi Send Message to Owner Message this owner
Hidden : 12/08/2020
2.5 out of 5
1.5 out of 5

Size: Size:   other (other)

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Geocache Description:

Park at the parking coordinates listed and access this earthcache from the Cross County Trail, under the Braddock Road bridge. No playing in traffic, please – use your head and DO NOT ATTEMPT FROM STREET LEVEL. Daytime access only, please watch out for other cycle and foot traffic and do not block the trail. Do not taunt Happy Fun Ball.

This earthcache will teach you about what materials make up the bridge above you, as well as how the features you see on the bottom of the bridge and on the concrete floor below formed. There is no physical geocache to find or paper log to sign. To log your find, you will need to read through the earthcache lesson below, walk along the path examining the bridge above and surfaces below, and make observations based on the reading and what you see. Depending on the lighting, a flashlight or cell phone may help you with your observations.

The bridge you are standing under was built in 1972. You may not have noticed, but parts of this bridge are starting to look a little like a spooky cave! These deposits, called calthemites, are a product of the bridge’s concrete breaking down over time.

To understand what we’re looking at here, let’s start with the basics. This bridge is made out of concrete, but what is concrete? Although the words concrete and cement are sometimes used interchangeably, they’re not the same thing – in fact, cement is a component of concrete. And “concrete” is used generally to describe the gray stuff you see at the coordinates, but it can also technically be used to describe asphalt. So let’s talk specifics.


Concrete is one of the most widely used resources on the planet. It takes three ingredients: water, aggregate, and cement. Aggregate is basically non-reactive filler, such as sand and gravel. Cement is what binds the aggregate together. Concrete is nothing new – ancient Greeks were making concrete floors over 3,000 years ago – but the recipe has changed. Let’s get cooking!

Recipe card for Portland cement

Now that we have our Portland cement, let's mix it with aggregate, and water. The cement reats to the water in a process called hydration, making a paste that in turn binds the aggregate together. The concrete sets when calcium oxide (CaO) in the concrete reacts with carbon dioxide (CO2), forming our old friend calcium carbonate (CaCO3) – especially at the surface, where the concrete is exposed to CO2 in the atmosphere. Not all of the calcium oxide inside the concrete mix reacts, though – more about that later.

We’re almost done making our bridge. But Portland cement concrete has a relatively low tensile strength and ductility – in other words, it can only take so much stress before it fails. Obviously, that’s not what we’re looking for in bridge material that needs to carry cars and trucks! In order to stand up to stress, Portland cement concrete is typically reinforced with something that can better handle that stress and help hold the concrete structure together. This bridge, like most modern concrete structures, is reinforced internally with steel cables called rebar. But these aren’t smooth cables – the surfaces are corrugated or ribbed so that it better bonds with the concrete.


Although concrete is very useful stuff, it’s not perfect. During the concrete manufacturing process, not all of the calcium oxide in the Portland cement mix reacts with carbon dioxide. Over time, water can seep into concrete, especially where it has cracked due to stress. This water weathers the concrete structure in two ways. First, it can oxidize the iron in the rebar, causing the metal to rust – and when it rusts, it can expand up to 4 times its original thickness, creating more cracks and even causing the concrete surface to fragment, or spall. If salt is used to melt road ice, the salt can dissolve in the water and speed metal corrosion. Second, the water can pick up calcium oxide from inside the cement, forming a solution called lime water (Ca(OH2)). When the lime water is exposed to the atmosphere, it reacts with carbon dioxide to form our old friend, calcium carbonate, which precipitates to form the hard calthemite structures you see here.


While what you see here might look similar to cave formations, this is not the same process that forms speleothems in caves, such as stalactites and stalagmites. In caves, carbon dioxide has turned rainwater acidic, causing it to dissolve limestone, and then the calcium carbonate precipitates out of the water. It is a slow process; it can take a speleothem anywhere from 100 to 2,000 years to grow just one inch. Since this bridge was built in 1972, you don’t need a ruler and a stopwatch to conclude that these formations formed a lot faster than that.

You may see multiple formations here: stalactites hanging from the ceiling, formed as dripping water reacts with CO2 in the atmosphere; popcorn-like coralloids, formed at fine cracks before water drops can form; and stalagmites, formed where water drips on the floor. On a sloping surface, stairstep-like rimstone, or gours, can form; micro-gours can also form on the edges of rounded stalagmites. Under the right conditions, calthemite structures can grow between a half inch to four or more inches in a year. However, uless the calthemites form in a spot that's protected from the elements, such as a bunker or basement, the fragile formations can break off from wear and tear of the elements.

You can tell about the mineral content of the calthemites from the color. Pure calcium carbonate calthemites will be white. Calthemites containing iron oxide can be yellow to dark orange or red. Calthemites containing copper oxide, from corroding wires or pipes, can be green or blue.

So, should you be afraid to drive over or walk under this bridge? It’s not in mint condition, but it’s still rated to be in fair shape. The damage you see is currently minor and can be repaired. The bridge is inspected every two years, with the next inspection to take place sometime around the publication of this cache – you can see the reports here.


To log this earthcache, send us a message or email us through our profile. Please copy and paste these questions, along with your answers – this helps us keep track of which earthcache you’re logging.

Please do not post the answers in your log, even if encrypted. There’s no need to wait for confirmation before you log a find, but we will respond if you send a message or include your email address in the email. Group answers are fine, please just let us know who was with you.

  1. The name of this earthcache: Calthemites at Accotink Creek Bridge
  2. Look up at the bottom of the bridge. What if any formations can you identify? Describe them.
  3. Is there a common factor to where these formations appear on the bottom of the bridge?
  4. Look down at the ground. What if any formations can you identify? Describe them.
  5. Based on the cache description and your observations, what mineral or minerals are present in these calthemites?
  6. Do you see any attempts to repair the damage under the bridge? If so, does it appear to have been effective?


Wikipedia: Portland cement, concrete, cement clinker, calthemite

Royal Society of Chemistry, Concrete stalactites

Buyers Ask, Rebar Problems in Concrete Foundations, Slabs, and Walls

Bridge Reports, Braddock Road over Accotink Creek

This earthcache was published with the approval of the Fairfax County Park Authority.

Nominated for Best Earthcache - thank you!

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