Stitching together rasters

[N R A]

Is there any easy way to 'stitch' together 2 (or more) rasters maps - 
this is mainly in regard to our use of r.watershed to produce an aspect 
map for the whole region. On our poor little Sun IPX doing it in one go 
would take forever, but I estimate that doing it as say about 4x10 
smaller blocks would only take about 40 hours. This is only really of any 
use if I can then somehow combine the small blocks back into one 'aspect' 
map.

Cheers,

Steve Culshaw
NRA North West
e-mail : nra at cix.compulink.co.uk (use this for general usage)
or         sculshaw at cix.compulink.co.uk (Private - only checked at best 
biweekly)