CALayer: add a border only at one side

I made a right border using this:

leftScrollView.clipsToBounds = YES;

CALayer *rightBorder = [CALayer layer];
rightBorder.borderColor = [UIColor darkGrayColor].CGColor;
rightBorder.borderWidth = 1;
rightBorder.frame = CGRectMake(-1, -1, CGRectGetWidth(leftScrollView.frame), CGRectGetHeight(leftScrollView.frame)+2);

[leftScrollView.layer addSublayer:rightBorder];