How does a depositional environment produce stratified sedimentary rock? Consider the example of the River Point Bar illustrated below. (If you are unfamiliar with stream morphology, ask your instructor, or look up this term in a Physical Geology text in the library.)
In (A) we see a simple diagram of a river Point Bar in cross section. The Point Bar is the sandy inner bank along a bend in a river, while the outer bank (known as the "Cut Bank") is usually steep or cliff-like because it is being eroded by the river.
As time passes (B), erosion causes the stream channel to move to the right, while the point bar builds out into the former channel, filling it with sand. The width of the channel stays pretty much the same, as does the width of the Point Bar, because plants continually colonize the edge of the bar furthest from the stream.
The result is a shifting of three environments (channel, point bar, forest) to the right over time. But as this shifting takes place, a record of their past presence has been left behind in the form of the sediment deposited by the stream. This record is a vertical record (as seen in (C)) which, if preserved, will eventually become a stratified sedimentary rock.
CREATING A MODEL
Does this mean that we would have to dig a hole in the forest floor as deep as the river channel in order to see what the stratigraphy would look like?
Actually, no. All we need do is walk from the forest to the edge of the stream (well, to be truthful, we should go for a swim in the stream too) and describe the sequence of sediments laterally to produce out model. Notice that our little stratigraphic column (from the top down) is labeled Green, Orange, Brown; while our lateral color-coded sequence of environments is Green (forest), Orange (Point Bar) and Brown (channel). By examining a sequence of environments we can actually predict the stratigraphy which will result. At this point we have created a model we can use to interpret the sedimentary rock record!