Fig. 16. Taverlone Mesa, New Mexico
Fig. 17. Monuments, Arizona
Note the rain furrows on the slope at the foot of the monuments. In the foreground are seen fragments of petrified trunks of trees, composed of silica and extremely resistant to the weather. On the removal of the rock layers in which these fragments were imbedded they are left to strew the surface in the same way as are the residual flints of southern Missouri.
Outliers and monuments. As cliffs retreat under the attack of the weather, portions are left behind where the rock is more resistant or where the attack for any reason is less severe. Such remnant masses, if large, are known as outliers. When flat-topped, because of the protection of a resistant horizontal capping layer, they are termed mesas ([Fig. 16]),—a term applied also to the flat-topped portions of dissected plateaus ([Fig. 129]). Retreating cliffs may fall back a number of miles behind their outliers before the latter are finally consumed.
