I'm new to GIS and have what may be a stupid question. I have two shapefiles, one showing the US states from 1789 and one showing the US states from today. I'd like to combine them into a single file, with a new field indicating the year. I've played with the merge function but that seems to merge them together into a single set of shapes, whereas I want to keep them as distinct sets of data, just in a single file. In SQL terms, I'm looking for a UNION ALL, as opposed to a UNION.