How to create columns in my drop-down menu?

Hi, I use the drop-down menu on top of the pages. It works just fine, but I have a lot of items under “Portfolio”. This makes the drop-down menu too long at some screens. So the lowest items are not visible anymore. I want to fix it. The best way to do it, is to make columns in the drop-down menu, I think. Can someone tell me how to do this? Or is it a better solution to make a scroll bar in the drop-down menu? And if so, how can I do this? Hope someone will help me, thanks in advance! Ellen www.kattenfotografie.com