As the rocks on our planet are continually being moved around by Plate Tectonics, it is rare to find layers of sedimentary rock still in their original position. Sedimentary rocks, like sandstone, mudstone, shale, etc are formed when sediments are deposited by water or wind and are generally formed in horizontal layers. But with the twisting and bending, uplift, and faulting over time those layers can be now at an angle, folded into ‘waves’ or even upside down!
One of the detective tasks of a geologist is to work out the original way the rocks were formed – and in sedimentary rocks that is working out which layer was deposited first, then next, and so on. And a piece of evidence they look for are sedimentary structures that show them “which way is up”.
There are many types of structures that can be formed in sedimentary rocks that will show which way is up. Here are just three:
If you take a jar of water and add some dirt containing rocks, sand and mud then close the top, shake it up then let it settle over time you will see that the biggest heaviest materials (rocks) will sink to the bottom, then the sand, then the mud – maybe over many days! This ‘grading’ from course to fine happens in nature as well! When those sediments become rock we can find that grading and it shows us which way is up – course up to fine.
In some locations, like off the offshore continental shelves, many layers of graded bedding can form – each one representing a layer formed from an event like a storm or even an earthquake when sediment was stored up and ‘flowed’ due to density down the slope onto the deeper ocean floor.
The action of waves and currents in a body of water can create ripple marks in the bottom sediments.
If these ripple marks get buried by new sediments, they can become preserved in the rock. These fossil ripples marks, due to their shape, show us which way is up – the ‘pointy’ parts pointing upwards.
When sand if moved in one direction by water, it forms a structure known as a delta front. Here the sand grains are washed down an angled slope. The next layer is washed down on the top of the first and so on. You wind up with layers of sand that are parallel, but these are at an angle to the sandstone bed. We call these cross-bedded sandstones.
The same process happens when sand of blown by the wind. The slope down which the sand falls is the front of a sand dune and you send up with cross-bedded layers in the sandstone.
The angle of the sand and the way it slopes is steep at the top then less steep at the end, showing us which way was up in the sand dune or the delta.
In some locations, the cross-bedding in sandstones can be very dramatic, such as these in Utah, USA.
Being able to work out which way is up can be so important when a geologist maps in heavily folded areas, like mountain ranges. Sometimes beds of sediments may be completely overturned so what may appear to be the top of the layers is actually the bottom!
Like many things in Earth science, we have to be detectives to work out what has happened – and way-up structures are just one piece of evidence in our Earth science tool kit!