Draw shadow only from 3 sides of UIView

I've successfully implemented drawing a shadow around my UIView like this:

block1.layer.masksToBounds = NO;
block1.layer.shadowOffset = CGSizeMake(0, 0);
block1.layer.shadowRadius = 1;
block1.layer.shadowOpacity = 0.7;

What now happens is I have a rectangular UIView and i would like to draw the shadow around it three sides, leaving the bottom side of it without the shadow.

I know that I have to specify the block1.layer.shadowPath by creating a new UIBezierPath but I'm not sure how to do it.

Obviously, setting layer.shadowOffset won't do the trick for me.

Thanks in advance!