As was noted in the previous article, it is perfectly possible for rocks to be overturned by tectonic processes. Hence, before we can apply the principle of superposition to discover the relative ages of the strata, we must first identify which way up the rocks were when they were formed.
Fortunately there are many indications we can use to find this out, known as way-up structures. In this article I shall list some of them
Note that I do not claim that this list is complete; these are simply some of the most commonly cited way-up structures.
Mud cracks (also known as dessication cracks) are formed in mud when it dries, and examples can be found preserved in the geological record. These form a distinctive structure, with their polygonal forms and the roughly V-shaped cross-section of the cracks; not only is there nothing else like them, but also there is nothing that looks exactly like mud cracks apart from going up where mud cracks go down. Hence mud cracks can be used as way-up structures.
Ripple marks and cross-beddingEdit
Ripples have curved troughs and sharp crests, and a convex shape as seen from above; as with mud cracks, we may note that there is nothing that looks exactly like a ripple only upside-down. Hence they form way-up structures.
The cross-bedding in aeolian sand is also convex on the upper side, as sand dunes are steeper at the top and have a shallower curve near the base (see, for example, the picture in the previous article).
Flame structures are formed when a denser sediment, (typically sand) is deposited on top of a less dense sediment (typically mud). The difference in density forces the mud to flow upward in what are known as diapirs, producing a distinctive flame-like structure in which the "flames" are always at the top. The photograph to the right shows an unlithified example.
(We may note in passing that diapirs are technically a counterexample to the principle of superposition: some of the earlier sediment has managed to rise above some of the later sediment. However, as diapirs are fairly easy to identify, they cause little confusion in practice.)
Some forms of deposition produce graded beds: for example, turbidity currents produce beds which grade upwards from coarse to fine material. Although occasionally reverse grading can be seen in turbidites, one typically finds turbidites in large stacks, so there is no difficulty in discerning the general trend and identifying the occasional example of reverse grading as being the odd man out.
Currents will often incise structures into the sediment over which they flow: flute marks, scour marks, sole marks, etc, producing distinctive impressions in the underlying beds which are then filled in with other sediments (we have discussed this particularly in our article on turbidites, but such structures are produced in other environments such as rivers).
In general, it is easy to look at a surface where two sediments meet and determine which of the two sedimentary rocks was eroded, and which was laid down over the eroded surface. If, for example, potholes formed by a river or glacier are subsequently filled up with mud, it is not difficult to conclude that the mud conformed itself to the potholes rather than the potholes to the mud, and that consequently the potholes indicate the lower surface.
Fossils attached to the surfaceEdit
Some fossils form attached to the ground, and display a distinct difference between up and down: so, for example, branching corals will branch upwards, not downwards; a tree-stump found still with its roots intact and embedded in seat-earth shows that the roots were down and the stump was up; stromatolites will have a flat base and a convex top.
Fossils not attached to the surfaceEdit
Even when fossils are movable and can be overturned, we can sometimes learn from them. For example, trilobites, being broad, flat creatures, are not readily overturned, and are usually found belly-side down as they were in life. Although one or two might get flipped over, if we have a fair number of trilobites we can distinguish the statistical trend, and say which direction was most likely up.
Bowl-shaped shells such as individual valves of bivalves will, in a current (such as that produced on beaches by the tide) tend to come to rest convex-side up, as the reader may easily observe by taking a stroll on a beach. Again, this will not be true of every single shell, but so long as we have a fair number of shells, we can gauge the general trend and use it to figure out which way was up.
The casts thrown up by invertebrates as they burrow are naturally found on the surface.
Some burrowing invertebrates make burrows which serve as way-up structures. For example, some make distinctive U-shaped burrows: naturally the openings are at the surface, so the prongs of the U point up and its bowl points down.
The indentations made by footprints are necessarily convex, and any sediment which fills them in will be concave, forming a way-up structure.
The photograph to the right shows the footprints of some large theropod dinosaur. Note also the mud cracks. Clearly this particular piece of rock is the right way up.
Geopetal structures are formed when a hollow object (such as a shell) becomes partly filled with sediment (such as mud). This provides us with a naturally occurring spirit level allowing us to tell up from down, and indeed the plane of horizontality, at the time when the sediment was deposited.
Bubbles in igneous rockEdit
When igneous rock is formed, bubbles of trapped volcanic gas will, of course, rise through the still-molten lava because of their lower density; for this reason, if we find bubbles in a solidified lava flow, they will tend to be at the top rather than the bottom.
Structures in lava flowsEdit
While the base of a lava flow will conform itself to the ground over which it flows, the top of the flow will usually not be flat, but may take on a number of distinctive forms: pillows if it was formed underwater, and aa or pahoehoe if it forms on land (the unusual terms are Hawaiian in origin).
The photograph to the right shows the distinctive "ropy" structure characteristic of the surface of pahoehoe: clearly this rock is the right way up.