"Systems of classification are not hatracks, objectively presented to us by nature. They are dynamic theories developed to express particular views..."
Stephen Jay Gould
At first it may seem that rocks are cold, hard facts over which there would be little disagreement. However, there is continuing disagreement over how to sort rocks into groups. Any classification of rocks reflects a theory of how the world is, and works. If the theory changes then the classification changes too. Numerous classification systems have been proposed to describe sedimentary rocks. Since 1950 over 50 classifications of sandstones have been proposed.What makes a good classification? First, it should be as inclusive as possible, incorporating as much variety as possible, leaving few or no orphan rocks. Second, it is internally consistent. All the rocks are classified by the same criteria and have equal status in the system, with no internal contradictions.
Geologists' experiences tell us that no one system is completely satisfactory. It seems that no matter what classification system is devised there are always specimens which do not easily fit into it. Also, the kind of classification a geologist uses depends on the type of study. A classification perfectly good for one study may be inadequate for another.
In this site we use a variety of classifiction systems that range from basic to sophisticated. For the most part, these are standard classifications common to introductory geology classes, or to the geological profession. And for each classifiction we provide an introduction that explains the theory behind the classification, and why is sorts rocks the way itdoes.
As you learn to classify and identify sedimentary rocks do not assume you are learning the "true classification". As you examine the rocks, continuously question the classification system you are using. Look for flaws and weaknesses, and strengths, in the classification. Seek out rocks which do not easily fit into it, and ask, "Why?", and, "How could the classification be improved?"
A classification is not only an end that we work for, it is also a beginning - a beginning of testing to find out if the classification in fact works in the real world. Or, a test to find out if the theory behind the classification is an accurate depiction of the world.