74 



EARTH FEATURES AND THEIR MEANING 



they must gradually disappear in rounded slopes under the action 

 of the elements. Smaller displacements within a rock which 



rapidly disintegrates under 

 the action of frost and sun 

 will likewise before long be 

 effaced. In those excep- 

 tional instances where a 

 resistant rock type has had 

 all altered upper layers 

 planed away until a fresh 

 and hard surface is ex- 

 posed, and has further 

 been protected from the 



FIG. 62. Post-glacial earthquake faults of small frost and SUn beneath a 

 but cumulative displacement, eastern New thin j of ^ itg QT[[ _ 



York (after Woodworth). " 



nal surface may be re- 

 tained unaltered for many centuries. Upon such a surface the 

 lightest of sensible shocks, or even the smaller earth movements 

 which are not perceived at the time, may leave an almost indelible 

 record. Such records particu- 

 larly show that the movements 

 which they register occur upon 

 the planes of jointing within the 

 rock, and that these ready 

 formed cracks have probably 

 been the seats of repeated and 

 cumulative adjustments (Fig. 

 62). 



Contraction of the earth's 

 surface during earthquakes. 

 The wide variations in the 

 amount of the lateral displace- 

 ment upon earthquake faults, 

 like those opened in California 

 in 1906, show that at the time of 



a heavy earthquake there must FlG< 63. Earthquake cracks in Colorado 



be large local changes in the deser 



density of the surface materials. Literally, thousands* of fis- 

 sures may appear in the lowlands, many of them no doubt a 



