(mostly) Newbie Questions Designer Pro X10

[handrawn]

If they are, you could make a rectangle the size of the area you want to keep and either select both and intersect shapes ...

which was what I was thinking - what you can do if they are uniform scans is align them all together by the two key edges - group them, and then you only need do one intersect, and then ungroup

otherwise a lot of individual cropping - all of which needs to be done before you even start what Frances suggests..