The sedimentary sequence seen in outcrops is the result of different types of sediment being deposited in different sedimentary environments over time, producing a vertical sequence of different facies.  Lateral changes in facies are relatively easy to understand.  Vertical facies changes may initially be somewhat puzzling.  How does one layer of sedimentary rock come to overlie another? The vertical relationships between facies are explained by changes in sea level, or changes in subsidence and sedimentation rates.

As laterally-adjacent sedimentary environments shift back and forth through time, as a result of sea level change, facies boundaries also shift back and forth. Given enough time, facies which were once laterally adjacent will shift so that the deposits of one environment come to overlie those of an adjacent environment.  In fact, this is how many (if not most) vertical sequences of sedimentary rocks were formed.  This concept was first stated by Johannes Walther in 1894, and is called Walther's Law.  Basically, in a conformable sedimentary sequence (i.e., one without unconformities), sedimentary units which lie in vertical succession represent the deposits of laterally adjacent sedimentary environments migrating over one another through time.

At any one time, sediment of different types is being deposited in different places.  Sand is deposited on the beach, silt is deposited offshore, clay is deposited in deeper water, and carbonate sediment is deposited far from shore (or where there is little or no input of terrigenous sediment).  Sedimentary environments (and facies) move as sea level changes, or as a basin fills with sediment.

A sea level rise is called a transgression.  A transgression will produce a vertical sequence of facies representing progressively deeper water environments (a deepening-upward sequence).  As a result, a transgressive sequence will have finer-grained facies overlying coarser-grained facies (fining-upward from sand at the bottom, and then to silt, and then to shale).  Transgressions can be caused by melting of polar ice caps, displacement of ocean water by undersea volcanism, or by localized sinking or subsidence of the land in coastal areas. 

A sea level drop is called a regression.  A regression will produce a sequence of facies representing progressively shallower water environments (shallowing-upward sequence).  As a result, a regressive sequence will have coarser-grained facies overlying finer-grained facies (coarsening-upward).  Regression can be caused by a buildup of ice in the polar ice caps, or localized uplift of the land in coastal areas.

Facies can also provide information on changes in sea level over time.  For example, as the shoreline retreats toward the land due to a rise in sea level, the marine offshore facies move toward the nearshore facies.  This particular shift in facies is known as transgression, and may be evidence of events such as subsidence or flooding.  When the stratigraphic record shows evidence of a shoreline moving away from the land, it is called regression. Regression occurs when there is excess sediment supply from the land which causes the shoreline to move seaward. Regression is often associated with times of tectonic uplift.