expected output from log of negative number

[E. Joshua Rigler]

If I type log10(-1), I get a complex number back whose real part is the 
log of the absolute value of the argument, and whose imaginary part is 
always equal to 1.36438.  What's more, I get similar behavior with a 
natural log, but the imaginary part is always equal to Pi.

I've managed to confirm that this behavior is compatible with ML, so I 
imagine it's not a bug, but can someone explain to me the logic here?