The animation that you are viewing was constructed so that the area of the elementary volume of sandstone was retained throughout the experiment. Pay close attention to the differently colored material lines. Do the lengths of some lines become shorter or longer as simple shear progresses? Do some lines rotate while others do not?
As you watch the animation of plane simple shear note that material lines shown in yellow become longer as strain progresses while the lengths of material lines shown in black remain unchanged. In contrast, the length of the material line shown in red becomes shorter. Note also that material lines colored yellow and red are rotated in a clockwise sense.