Anomalies in slopes derived from 30m. DEM

Background
As described elsewhere ISE recently completed assembly of a 30m. 
Digital Elevation Model  for the Willamette River basin from  a collection 
of individual raw, ASCII 7.5 minute DEM's maintained by the Oregon State 
Service Center for GIS.  In producing various slope maps from the basin-wide
DEM, we have noticed some anomalies which are not clearly related to the known 
sources of artifact within or differences between the 7.5 DEM's.  In this map 
of degrees slope, the center two quadrangles, Peoria and Harrisburg, show
a level of detail in red that is significantly reduced at the boundaries with
adjacent quads.  In this treatment the focus (red) is on slopes of between one
and two degerees.  White represents slopes between zero and one degree, and
blue represents slopes greater than two degrees.

In the comparison of Greenberry with Peoria, for example, the metadata show
that both have quality level two, both have vertical RMSE of 7, and both
were originally delivered in vertical units of meters.  Additionally, the 
header records of the two original files indicate that both were produced by
vectorization of scanned contour maps using LTPLUS.

If the focus is on the slope range greater than zero and less than or equal
to one degree, the anomalies are more clearly related to contours as in this
image.  Red in this image depicts the focal slope range (0 - 1 deg.).  The
detection of terracing does not, however, explain the difference visible 
between Greenberry and Peoria when both were apparently produced in the same
manner.