Page:The Journal of geology (IA journalofgeology311923univ).pdf/710

618 ington, north of Snake River. These scablands have a history which is believed to be unique. The prevailing feature of their topography is indicated in the term here used: channeled scablands. They are scored by thousands of channels eroded in the underlying rock. The plateau in Washington, north of Snake River, has a total area of about 12,750 square miles, of which at least 2,000 square miles is channeled scabland. The scabland is widely distributed over the region in linear tracts among maturely dissected hills which bear the loessial soil (wheat lands) of the plateau.

The following features and relations of the scablands exist in all tracts. They must form the basis of any interpretation for the origin of channeled scabland. The map should be examined as this list is read.
 * 1) Scabland tracts are developed invariably on or in the Columbia basalt formation.
 * 2) Scabland tracts are invariably lower than the immediately adjacent soil-covered areas.
 * 3) Scabland tracts are invariably elongate.
 * 4) The elongation of scabland tracts is with the dip slope of the underlying basalt flows. There are eight known exceptions to this rule, all minor affairs so far as length is concerned.
 * 5) Scabland tracts, considered as units, invariably have continuous gradients.
 * 6) Scabland tracts are invariably bounded by maturely eroded topography.