WEATHERING 



31 



Creep of Soils. — Another important result of frost action is the 

 creep of soils on slopes. If soil contains water, each freezing slightly 

 raises the fragments 

 at right angles to the 

 surface of the hill, 

 and each thawing 

 permits gravity to 

 pull them down hill. 

 If the process is often 

 repeated, the soil 

 moves slowly down 

 the slope. In the 

 course of many years, 

 many tons of earth 

 may be thus carried 

 to a lower level. 



2. Changes in 

 Daily Temperature. 

 — In regions where 

 the air is dry and 

 clear the radiation 

 of heat is rapid and 

 the range in daily 

 temperature is wide, 

 often varying 8o° F., 

 while in the Sahara 

 Desert a change of 

 13 1° F. within a few 

 hours has been re- 

 corded. In such re- 

 gions, the naked rocks are heated to a high temperature during the 

 day and are cooled rapidly at night. Since rocks are not good con- 

 ductors of heat, the side of the rock exposed to the sun's rays is often 

 raised to a temperature of 120 F. or more during the day, while a 

 short distance beneath the surface the rock is still cool. The result 

 is that the outside shell is expanded, while the interior is still con- 

 tracted. Strains are thus produced which tend to break off fragments 

 of the rock, dark-colored rocks being particularly affected, since 

 they absorb more heat. In the late afternoon and night, on the other 

 hand, when the temperature falls, the interior which had been grad- 



CLELAND GEOL, — 3 



Fig. 7. — Rock glacier, McCarthy Creek, Alaska. The 

 talus forming the rock glacier is derived from the high 

 cliffs (cirque). (U. S. Geol. Surv.) 



