Problem solved!
Firstly, Column is the correct word. Newspapers and magazines have several vertical columns to make text easier to read. The word row isn't used by printers when referring to text. it's simply lines.
Spreadsheets have vertical columns numbered 1 - 50 across the page and horizontal rows numbered A - Z down the page. (or is it the other way around?) The word row is used in this instance because the columns in each row may contain several lines of data. Thus we can refer to lines or rows whenever necessary.
The Answer:
Page Builder won't allow you to add an additional column to a full column. To create columns with adjustable widths, you must use the Add Column button.
To overcome this quirk, simply do this. It takes ten seconds:
Create a new column it will appear under the full column containing Timeline.
Create a second column. This new column will appear alongside the previous column you created. Thus you now have two columns. Add a third if you wish.
Drag the Timeline block(s) into any of your new columns. Adjust their size.
Delete the original full width column and the job is done!
To sum it up, full columns are a fixed width determined by the top slider. Ordinary columns are fully adjustable by moving the slider immediately below them.