r.aspect from USGS DEM data

[FINNEYB at AXE.HUMBOLDT.EDU]

I have imported 6 quads of 7.5min USGS DEM data (30m x 30m grid) into GRASS4.0.
r.slope.aspect was run on the elevation data.  When displaying the raster
image of the aspect data, there are vertical black bands (no data?) running
along the east and wast edge of the quads (where two quad maps would join).
However, where two maps join on the vertical direction, the aspect raster
map does not have a black band.  The width of the "no data" band is about
300 meters (10 cells).  The DEM data was read with m.extract.dem from a
single tape.  Anyone have a clue as to the cause of the black bands, and
how I might get rid of them (or smooth over them)?
 
Thanks in advance
 
Brad Finney
Environmental Resources Engineering, Humboldt State University, Arcata, CA
finneyb at axe.humboldt.edu