Describe two different methods of correlating rock units. How was correlation used to develop the geologic column? What is a stratigraphic formation?

Lithologic correlation is a method of correlating rock units based on their physical and chemical properties, such as rock type, grain size, and mineral composition. This method assumes that rocks with similar properties were formed under similar conditions and are therefore related. To perform lithologic correlation, geologists usually study rock samples and create detailed descriptions of the rock units. Then, they compare these descriptions to find similarities and establish correlations between rock units in different locations. This method can be limited by the fact that two rock units with similar properties might have different ages, so additional evidence is often required to confirm the correlation.

Fossil correlation is based on the principle of faunal succession, which states that specific groups of fossils appear and disappear in a consistent, recognizable order throughout the geologic record. This method involves comparing the types and assemblages of fossils found within rock units to determine their relative ages and correlate them across different regions. Fossil correlation is particularly useful when the rock units do not have unique lithologic characteristics or when the rock units have been extensively altered or metamorphosed. By identifying the evolutionary sequence of fossil assemblages, geologists can more accurately determine the relative ages of the rock units and establish correlations.

Correlation, both lithologic and fossil, was used to develop the geologic column. The geologic column is a representation of the Earth's history through time, showing the sequence of rock formations and their relative ages. It is based on the principle of superposition, which states that in an undisturbed sequence of rock layers, the oldest layers are at the bottom and become progressively younger towards the top. By using correlation methods, geologists were able to compare and match rock units from different locations and arrange them in a chronological order. This led to the construction of the geologic column, which is now considered a fundamental framework for understanding Earth's history and the evolution of life on our planet.

A stratigraphic formation is a mappable, distinct rock unit or succession of rock units that have unique lithologic, fossil, or other characteristics and can be distinguished from the surrounding rock units. Formations are the basic units of stratigraphy and are used for correlation and interpretation of geologic history. They are usually named after a geographic location where the formation is well-exposed and can be easily studied. Stratigraphic formations can range in size from tens of meters to several kilometers in thickness and can be composed of sedimentary, igneous, or metamorphic rocks.