Why do sand dunes, hills and mountains assume the shape they normally do? What about something taken in a tablespoon? This shape, as I understand, could be explained by gravity, friction and similar concepts, but why the shape isn't cubical or cylindrical couldn't be explained in easier terms. Please help me sort out this conundrum.
 A: The angle of repose of a granular material describes the shape of a pile of sand. When the pile is in mechanical equilibrium, the shear stress nowhere exceeds the shear-strength of the sand on any arbitrarily oriented plane. The tensile strength of the sand is usually very small. The constitutive law describes the dependence of the failure criteria upon the normal and shear stress on an arbitrarily oriented plane. For simple geometries this may be sufficient to calculate the shape of the sand pile subject to gravity and forces such as fluid flow.
A: Let's think about fluids, and in particular the ocean.  The deeper down you go, the more pressure you feel.  This increasing pressure is a result of the increasing amount of water that is above you, weighing the water down.
With this in mind, let us suppose that a table spoon of milo is scooped up with the milo forming a cylindrical shape.  Now the "pressure" on the milo at the bottom of the cylinder will be greater than at the top, and consequently it will have a greater tendency to disperse. 
Therefore, since the lower layers disperse more than the upper layers, a dome shape is formed in the spoon.
Once we know the shape won't be cylindrical, and therefore sloped, we can work out how the slope's shape is determined.  
If we imagine a stream of sand falling gradually onto a mound, initially particles at the top of the slope won't roll down because the slope is too small to overcome the opposing force of friction, and the height of the slope will build.  As the height and slope of the hill increases, it reaches a critical height where the sand's friction can be overcome by the slope, and this will be the maximum slope formed, known as the repose angle.  This angle varies from substance to substance.
A: Because the particles are loose in Sand or substances like it  . 
But if you use wet sand then you may be able to shape it in cubical form . This can also be seen in table salts during moist or humid weather conditions , the salt usually in side the container usually forms clusters and it becomes difficult to take it out .
