The horizontal
axis is <i>x</i> and the depth of snow is indicated in blue.
The plows are small circles running on the <i>x</i> axis.
The pictures are 8 minutes apart: the time of each is indicated.  
The animation begins at 12:00 when the first plow starts at the left.
At 1:00 the second plow starts at the left, plowing the snow that has fallen
since the first plow passed.  The animation ends when the two plows 
meet at 2:00.

<p>
<img src="plowani.gif">

<p> To restart the animation, use the "Reload" button on your browser.