multiple columns of data stacked into one column

Does anyone know how to do this? I have lots of columns that need to be stacked into a single column (I do not want to combiner or concantenate)
eg put all data in column B and column C and stack them one under each other into a single column in column A. I have many many columns each with approx 300 rows of cells.
Any ideas appreciated, I have looked in all the forums but cant quite find the right answer.

Thanks

Beatrice