SEED Science

Stratification & Segregation
Ancient Sand Dunes

Wind-blown sand often forms into dunes - alternating hills and valleys that look like waves. This diagram (right) represents a side view of a cross-section of an ancient dune that has solidified into sandstone. With the wind blowing from the left, grains moved up the slope and then slid down the slip-face. The descent was much like what happened when we poured sand mixtures into the Hele-Shaw cell.


mD stands for milliDarcy, a unit of permeability

The dotted rectangle is a cross section of that dune,which is shown in the image on the left. This image was produced by testing the permeability of a slice of rock 170 millimeters wide by 450 mm high. Permeability is a measure of how rapidly a fluid - a liquid or a gas - flows through a porous solid like sandstone. The brighter, orange and yellow areas have higher permeability than the brown and black areas. Sandstone made up of larger grains will tend to be more permeable that fine-grained sandstone. In this particular rock the grains are segregated by size into layers, just like in our experiment.

Find out more

In the Science Lab you'll find instructions on how to build and use the Hele-Shaw cell used in the Stratification & Segregation experiments. There are other experiments and demonstrations to explore how mixtures of different kinds of grains can be separated out again. It may not be quite like getting potatoes back from mashed potatoes, but you may be surprised at what you can unmix. Click here to go to that activity.