Here's my take on it;

After a quick read through of sailor_ed's, I think that I'm crossing some of the same ground. Anyway, I used the Vertex Modeler, inserted a cylinder [20 sides, 1 segment]. Then a smaller cylinder. [4 sides, 11 segments] Squeeze the 'square cylinder' flat. Select points on one side, one section at a time, and make the shape taper to a thin wedge at the top. Inset this into the column. I made it go all the way through, but it's not necessary. Now select all points and duplicate. If you do a boolean subtraction on one set, you'll have a column with a wedge-shaped hole. Do a boolean intersection with the other, and you'll have a perfectly matching plug. By inserting the plug, and reducing the size on one axis, you can make a crack as deep as you want running up the side. In the last pic., I made the wedge very wide. Not you can build a column with a piece completely cracked off, or slid down, or about to.