In the north-west it includes spurs of the Taunus.
The south slope rises precipitously from the foothills; the north slope is more gradual, but it is much broken by rugged spurs and deep gorges.
It has been suggested that these spurs represent the sites of vents or fissures of eruption.
It extended the meaning of the term " railroad " to include switches, spurs and terminal facilities, and the term " transportation " to include private cars, and all collateral services, such as refrigeration, elevation and storage.
By the rain wash and wind action detritus from the mountains is carried to these valley floors, raising their level, and often burying low mountain spurs, so as to cause neighbouring valleys to coalesce.