A formation in sedimentary geology is a unit of rock that is consistent and identifiable throughout a region. Formations are identifiable not only by the type of rock, but also by index fossils that serve to identify the unit. One well known example is the Green River Formation of Wyoming which is a particulary rich one for fossil collectors from the Eocene Epoch.
Formations are vital in well-drilling, because some formations are consistently good aquifers or oil or gas bearing strata. An example of the value of studying the formations of an area would be the water-well driller who may cease drilling at a point, though he might wish for larger quantities of water, because he does not want to contaminate the water he now has at his disposal, with deeper lying formations known to contain salt or sulfur compounds.
High speed drill bits used in deep wells, grind the rock to very fine gains or even powder, but microscopic examination of them is still able to identify small marine organisms or pollen.