Invasive Rocks and an Unstable Bluff EarthCache
Invasive Rocks and an Unstable Bluff
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This earthcache is located in the City Point area of Petersburg National Battlefield. There is parking nearby at the corner of Pecan and Water streets. The terrain is mostly level, but somewhat uneven and sloping near the shore. An easily discernible path leads from the parking area to the cache location.
The City Point section of Petersburg National Battlefield is home to some very peculiar geologic interlopers. At various places along the shore there are scatterings of small stones. Some of these are not native rocks and are invasive, as it were. The invasive stones are flint nodules. There are no significant flint deposits anywhere near City Point. Indeed flint is a rarity in the eastern United States. In prehistoric times flint was greatly sought after by many groups of Native Americans and some eastern people traded across the continent to get it from the Rocky Mountains and beyond. The flint nodules on the shores of the James River at City Point are invaders from Europe. These flint nodules were brought to coastal Virginia by ship. When British ships came to supply the colonies they were carrying mostly finished manufactured goods, most of which were not very heavy. Consequently the ships needed ballast to maintain stability in the open ocean. The flint nodules were used for ballast. When the ships arrived in Virginia they off loaded their goods. The lumber, tobacco, and other raw materials that were loaded up for the return trip to England were generally much heavier and took up much more room. These goods served as their own ballast and the flint was dumped in the river. During subsequent floods and dredging operations the flint nodules were pushed toward shore and can now be found many places along the banks of the James.
So you know sort-of what to look for; Flint does not have a specific color, but is often dark gray with shades of brown, red, or yellow, and sometimes white. Brighter or more colorful variants are sometimes called chert by some people.The color can be caused by inclusions of organic compounds (black), metal sulfides (black), and various metal oxides and hydroxides (yellow, orange, brown, reddish, etc.). It is slightly translucent to almost opaque, sometimes only thin chips are translucent at the edges. On freshly broken surfaces the luster is dull, at best waxy, but because it is very hard, flint takes a good polish and assumes a glassy luster.
Another interesting geologic feature of this location is the City Point bluff. The top of the bluff is occupied by the Epps House and was the site of the Gen. Grant’s headquarters during the Siege of Petersburg. The bluff itself is of interest because the upper layers are composed of a very sturdy clay, which does not easily erode. However, the lower part of the bluff contains layers of sand, which were laid down when what is now eastern Virginia (as well as most of the rest of the east coast) were underwater. The higher sea levels of the late Oligocene and early Miocene Epoch flooded this area with seawater until about 5 million years ago. The sandy sediment made the bluff unstable because as the sand was exposed and eroded out sections of the bluff crumbled into the James River. Several years ago the National Park Service completed a ‘Bluff Stabilization Project’ and constructed barriers to keep the bluff from eroding and endangering the historic structures resting on it. The largest of the retaining walls is visible a short walk upriver from here.
To log this cache you will have to email me the following:
1.The number of people in your group
2.Estimate the elevation of the bluff.
3.What is the approximate height of the retaining wall of the ‘Bluff Stabilization Project’?
4.Look at the stones on the shore. About how large are the flint nodules?
Optional but appreciated - Post a picture of yourself or your GPS with the river in the background.
IMPORTANT: It is illegal to remove anything from National Park lands. Leave any stones, vegetation, or other items where you found them. Take nothing but pictures.
Additional Hints
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