The line shows you the type of fold (mountain or valley) and the arrow tells 
you the directionality of the fold (fold top to bottom or fold the bottom up to 
the top).  
 
The different type of arrow head is just extra information to help distinguish 
between the two.  Like, for example, traffic lights. Not only is red for stop 
and green for go, there is also positional information (red is on top, yellow 
is in the middle, green is on the bottom).  Or stop signs: red for stop, the 
letters STOP for stop, and the octagonal shape for stop.
 
That's my guess, anyway.
 
Diana

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