61.1: Introduction

“Which way’s up?”

With rocks, the answer is not always clear.

If layered rocks have experienced mountain building, they may be rotated from their original horizontal positions into vertical orientations, potentially confusing geologists, since the principle of superposition no longer applies.

Figure \(\PageIndex{1}\): An exciting outcrop: but in order to interpret it correctly, we need to know which strata are older, and which are younger.

Consider the hypothetical example illustrated in the image at right: you discover a vertical sequence of strata. In one layer, you find evidence of a mass extinction event. In another layer, you find evidence of glaciation. Did an ice age cause the extinction? Or did the extinction somehow trigger the ice age? In order to pose intelligent questions about causality, you need to know which one is older. The older event can influence the more recent event, but not the other way around.

Even more extreme is when compressive stresses associated with convergent plate tectonic settings manage to fold beds into up-side-down positions. If the beds are up-side-down in such a tectonic inversion, it could really throw off the interpretations of the Historical Geologist: they would be reading Earth history backward!

Figure \(\PageIndex{2}\): A tight, recumbent fold in the Sierra de Juarez, Mexico, shows both upright and up-side-down bedding. Way-up (or “geopetal”) structures allow us to tell the two apart.

In such situations, we need reliable tools in order to accurately interpret which direction was “up” when the rock originally formed. We call these patterns that look different right-side-up compared to up-side-down by the general term “way-up structures.” Some geologists also call them “geopetal structures.”

Way-up indicators are critical for figuring out the correct sequence of geologic units. The help us determine “younging direction,” the direction in which strata get younger. (This is the same as paleo-“up” or “facing direction.”)

The welter of terminology shouldn’t turn us off: it’s an indication that geologists put a strong emphasis on finding and relying on way-up structures. Consider this example:

Figure \(\PageIndex{3}\): Geopetal structures that point in the paleo-“up” direction (red arrows) are of critical value in deciphering the story told by a sequence of strata.

Three layers are shown in outcrop at Earth’s surface, color-coded green, blue, and yellow. They are folded into what appears to be a series of anticlines and synclines. Given superposition alone, we would assume the lowest one is oldest, and the uppermost one is youngest. However, way-up structures in these three units point downward as the “up” or younging direction. Therefore, the exposed layers are part of a larger-scale fold, and the upper (upright) portion of that fold has been removed by the forces of erosion. Without the way-up indicators, we would have been fooled. So they are very important.

So what are these geopetal tools, exactly?

The key thing is that a way-up structure must be display some difference between its top and its bottom. They always look different up-side-down compared to right-side-up. In sedimentary rocks, the following way-up structures can aid the historical geologist in figuring out the paleo-“up” direction:

• cavity fills
• crossbeds
• ripple marks
• mudcracks
• graded beds
• loading structures
• sole structures
• burrows
• stromatolites

In igneous rocks, there are fewer options, but a few that are handy include:

• vesicle concentration
• apophyses
• pillow structures
• in a few rare intrusions, primary structures including grading and cross-bedding(!)

Metamorphic rocks develop no way-up structures as a consequence of metamorphism, but sometimes primary structures in a sedimentary or volcanic protolith can potentially survive as discernible patterns in lower-grade metamorphic rocks. Higher-grade metamorphic rocks are so thoroughly recrystallized that any original geopetal structures would be destroyed.

A note of caution

Be cautious about leaping to big, important conclusions from a single way-up structure. The way-up indicators shown on this page are hopefully straightforward, but nature is vast and varied. Examples seen in the field can frequently be ambiguous. The careful historical geologist will search for as many examples as possible; to see if they agree with one another. Careful geologists seek a preponderance of evidence before settling on an interpretation.