20O PRINCIPLES OF STRATIGRAPHY 



generally below the influence of the seasonal variation, which in 

 temperate regions may extend to a depth of 50 feet. The normal 

 temperature of spring water lies in general between 47° and 51° F. 



Freezing of Riz'ers. Rivers of comparatively gentle current will 

 freeze over after the manner of lakes, but rivers of strong current 

 will freeze over only under exceptional circumstances. In all cases 

 the current continues under the ice. 



Freezing of Ground Water. The ground water will sometimes 

 freeze to an astonishing depth. Thus in the Tundras of Alaska and 

 Siberia the permanently frozen soil often extends to a great depth, 

 that of Yakutsk, Siberia, having been given as 382 feet. (K. E. von 

 Baer, quoted by Russell-31 '.130.) Sands saturated with glacial 

 waters may freeze throughout and then behave like solid rock. In 

 this manner faulting, crumpling, and other structures normal only to 

 solid rock have been formed in the otherwise unconsolidated glacial 

 sands and clays of the glacial period. (Berkey & liyde-i : 223- 



231-) 



Mechanical zvork €f freezing ground zvater. (Salisbury-34: 

 208.) The upper layers of the lithosphere in high latitudes are 

 subject to periodic refrigeration below the freezing point of the 

 ground water. This is especially true of the soil layer, which may 

 be affected to a depth varying considerably with the latitude, the 

 intensity, and above all the duration of the period of cold. When 

 the soil is frozen, erosion is retarded, and where the subsoil is 

 permanently frozen, as in the northern regions of the larger con- 

 tinents, the arboreal type of vegetation is absent as already noted. 

 The freezing of the soil also disturbs the solid particles in it. Thus 

 Stones and boulders work their way up through the soil to the 

 surface, producing constantly recurring stony surfaces, leading 

 sometimes to the belief that stones "grow." Foundation walls of 

 buildings which do not extend below the zone of freezing are like- 

 wise disturbed in this manner. 



Moisture rising from the soil through capillary attraction may 

 freeze as it reaches the surface and so form ice crystals which push 

 upward by addition from below. These may be two or three inches 

 in length, and they will raise leaves, sticks and stones. 



Where the soil is thin the water may freeze in the crevices of 

 the rock beneath the soil cover, and so shatter the rocks. This is 

 especially effective where moisture is abundant, and where there is 

 a frequent change of temperature from above to below the freezing 

 point. (On the depth of frost in the Arctic regions see Wood- 

 ward-45.) 



Thermal Springs. Heated waters reaching the surface in the 



