December 9, 2012
Loose pieces of rock that are not connected to an outcrop.
If you’re in the field with a geologist, you’re very likely to hear the word “outcrop” and the phrase “in situ“. When describing, identifying, mapping, and understanding rocks, geologists like to see rocks in context. If rocks were alive, you might say that geologists like to observe rocks in their natural habitats. You might say that geologists like to observe where rocks live and who their neighbors are and how they interact with their neighbors. Of course, rocks aren’t alive, but geologists still find it very useful to observe rocks in situ, a Latin phrase that literally means “in position.” When rock is observed in situ, that means that it is attached to an outcrop, which is a place where bedrock or other “in position” rock is exposed at the Earth’s surface. Sometimes, outcrops are natural– they are places where weathering, erosion, faulting, and other natural processes have exposed hard rock above softer soil, sediment, alluvium, and colluvium. Often, outcrops are manmade. Geologist are found of observing rocks exposed at manmade outcrops such as roadcuts and quarry walls. Observing rocks in situ at outcrops allows geologists to gather much more information about the rocks than can be gleaned from a fragment of rock alone. By observing rocks in context, geologists can gather much information about the structure, stratigraphic position, size, degree of weathering, and many other aspects of a particular body of rock. Observing rocks in situ at an outcrop is particularly important for geological mapping. Only rocks observed at an outcrop can be confidently delineated on a geologic map.
When geologists encounter pieces of rock that are not found in situ at an outcrop, they refer to these rocks as “floats.” Floats are pieces of rock that have been removed and transported from their original outcrop. Sometimes, float rocks are found very close to outcrops. For example, weathering and erosion may create a pile of float rocks at the bottom of a hill below an outcrop. Often, geologists will first notice float rocks and then will look around– and often find– the outcrop from which the float rocks originated. Of course, geologists can never be 100% sure that a float rock originated from a particular outcrop, but they can be pretty certain if there is a similar rock in outcrop nearby the float. Other times, float rocks are found very far from their original outcrops. Water, ice, and even wind can transport rocks very far from their original outcrops. A well-known type of float rock is a glacial erratic, a rock which has been scraped up and transported by a glacier.
Float rocks can even be transported by anthropogenic activities. Many rocks are quarried and used for buildings, walls, roads, bridges, and other construction projects. Anthropogenic activities can move rocks far from their original outcrops. For example, in rural New Hampshire where I grew up many of the roads are gravel roads. The gravel that covers the roads is quarried and brought in by truck. I like to walk along the gravel roads near my parents’ house in New Hampshire and pick up interesting pieces of gravel. Sometimes, the gravel pieces contain spectacular garnets, micas, and other pretty minerals. I often find myself wondering about the geology of these gravel rocks. I can understand some things about the geology of these gravel float rocks, but to really understand these rocks I’d need to track down the quarry locations and go look at an outcrop or two.
Often, geologists are brought float rocks to identify. Curious non-geologists often pick up loose pieces of rock and bring them to geologists for identification. Commonly, people pick up dark-colored rocks and wonder if they are meteorites (most often, they’re not). Whenever I am brought a float rock (or am sent pictures of a float rock), one of the first questions I ask is, “Where did you find the rock?” I also often ask, “Were there any outcrops of the rock nearby? I mean, places where the rock was still attached to the Earth?” Often, the reply to these questions is, “No, I just picked up the rock. I don’t really remember where– somewhere in such and such place.” I do my best to identify float rocks when I can, but the truth is that there is only so much information that a geologist can gain from a float rock. Don’t get me wrong– geologists can still learn a great amount from float rocks. Nevertheless, geologists prefer to observe rocks in their natural habitats.