This is an Earthcache – as such, there is no physical cache. Instead you will partake in a geology lesson by making observations and sending me the answers to 6 questions about the limestone boulder at the posted coordinates. Permission for this placement was given by the Tourism Council of Frederick County.

THE BRADDOCK MONUMENT

Facing north along eastbound Alternate Route 40, east of the crest of Braddock Mountain, is a roadside monument composed of a bronze tablet on the face of a large boulder taken from the mountain. In 1932 the Frederick Chapter of the Daughters of the American Revolution commissioned this monument and plaque that reads This boulder marks the National Trail over which traveled Gen. Edward Braddock and Lt. Col. George Washington, 1755.

BRADDOCK MOUNTAIN LIMESTONE

Frederick County lies within two of the five geologic provinces located in Maryland, the Piedmont Plateau and the Blue Ridge. The rock type in the Frederick Valley is Frederick and Grove limestones with some diabase intrusion and New Oxford Formation overlying the limestone at its western edge. Its sedimentary rock is easily eroded to form deep soils, whereas the metamorphic and especially the igneous materials of other regions require more time and more severe eroding.

Having abundance of natural limestone is evidence that the area was once under ocean water, long before the tectonic plates collided. Limestone forms in warm, shallow seas when generations of tiny sea creatures die and accumulate on the ocean floor, later to compress into solid rock.

Limestone is a sedimentary rock that is made up of at minimum 80% calcium carbonate (CaCO3) and is formed in layers. Pure limestone appears white or almost white, but usually the layers of limestone contain other minerals that might include clay minerals, micas, quartz, pyrite, iron oxide, graphite, and other impurities that turn them a variety of colors from yellow to gray to red.

Looking the side of this boulder you can see some very thick bedding planes. These layers represent periods of deposition, between which the conditions changed. For example, between two layers of limestone, there is often a thin seam of shaley material, registering a time when clayey matter was worked off from the shore. If the layer is very thin (less than one centimeter or 0.4 inch), it is called a lamina.

Limestone is particularly susceptible to chemical erosion due to that it is prone to dissolution by acid rain. Sulphur dioxide is an atmospheric gas which often result in a loosening of the exterior of the stone causing loss of material. Cracks and joints on bedding planes make the rock permeable to water and ice, both physically altering the surface of the stone and trapping water inside.

LOGGING REQUIREMENTS:

QUESTION 1. Frederick County lies within what two geologic provinces?

QUESTION 2. Mica is the mineral responsible for putting a sparkle on many rocks. Are shiny specs of mica present in this limestone?

QUESTION 3. Looking at the east side of the boulder you can see a clean cut of the vertical layers. About what size are the the thicker dark banded layers on the right side?

QUESTION 4. Would you consider these dark banded layers to be lamina?

QUESTION 5. Look at top the long vertical crack on the east side in the center of the layers. What weathering do you think caused that?

QUESTION 6. At top the long vertical crack it appears repairs have been made to stop or slow down the weathering. Do you think this patch will successfully hold back the weathering that caused the crack?

OPTIONAL PHOTO: Posting a photo in your log readily indicates that you (and anyone else logging the find) are at the location.

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