In this article we shall look at what Walther's principle is and why it works.
Walther's principle can be stated as follows: If sediment A is succeeded vertically by sediment B without an unconformity between them, then sediment A will also be succeeded horizontally by sediment B in some direction.
The somewhat abstract statement of the principle will be clarified by a few examples.
Walther himself was led to formulate his principle by looking at fluvial rocks. As a meandering river shifts its course, the riverbed (consisting, for example, of gravel) is overlaid by the sand of a point bar and then the mud of a flood plain. But this is exactly the same sequence as one would see moving laterally outward from the center of the river out to the bank: gravel, then sand, then mud. It is this lateral succession of sediments, plus the fact that the river shifts, which causes the vertical succession in a fining-up sequence, so naturally they are going to be the same.
Similar effects are caused by shifts in sea level. Consider the various depositional environment and their associated types of sediments (facies) that we would see on the sea floor as we move further from the shore and out to sea: a typical progression would be sand grading into mud which grades into calcareous ooze.
Now consider what happens if the sea level rises and so the shoreline moves inland (a marine transgression). The sandy facies would move landwards; so would the muddy facies, and similarly with the calcareous ooze. Now this means that in some locations we will see a vertical succession of sand giving way to mud; and further seaward a succession of mud giving way to limestone.
In a marine regression, when the sea level falls, the facies would of course move in the opposite direction, reversing the vertical succession.
The reader should note that this is why finding a physically continuous rock formation (of sandstone, for example) does not indicate that it was all laid down at the same time. The seaward end of it could have been laid down at the beginning of a marine transgression, and the landward end of it at the end of the transgression. So long as the transgression was sufficiently gradual, this would produce a continuous layer of sandstone (perhaps sloping slightly upwards from the seaward to the landward direction) and yet one end of it would have been laid down at a different time from the other. Such a transgression can be detected in the geological record because although the sandstone will remain sandstone throughout its length, the fossils at the landward end will be younger than those at the seaward end.
As a final example, consider the deposition of marine sediment on a moving oceanic plate. A particular section of the plate moves (let us say north, for example) from a location where pelagic clay is deposited to a location where calcareous ooze is deposited. Then it follows that when mud was being deposited on that part of the plate, calcareous ooze was being deposited to the north of it, and when calcareous ooze is being deposited on it, pelagic clay will be deposited to the south of it. The horizontal succession from south to north is pelagic clay followed by calcareous ooze, which is just the same as the vertical succession from bottom to top.
Walther's principle: how do we know?Edit
The principle is a simple one: all it does is tell us what we should see if we look; and we do in fact see what the principle tells us we ought to see.
What's more this is what we ought to see in theory, because as you can see in our examples it is the horizontal succession of sediments that causes the vertical succession of sediments, so naturally the two successions are going to be the same.
The principle can have exceptions, but they will necessarily be rare. If there is sudden shift in the mode of deposition (such as a tsunami rapidly laying down a bed of sediment known as a tsunamite) then because of the underlying mechanism the succession of sediment will not obey Walther's principle. However, such an event will tend to be so violent in its nature as to be erosional and to produce an unconformity; and so it would not constitute a counterexample to Walther's principle, which by definition only applies to beds of sediment without an unconformity between them.
It is usually the case that a change in the mode of deposition at a given place will correspond to a gradual geographical shift in the places in which different types of sediment are deposited (or, as in our final example above, a shift of the geography itself beneath the different areas of deposition). For this reason Walther's principle will be generally if perhaps not universally applicable.