PHYSICAL GEOGRAPHY 261 



water inside the knolls can rupture a greensward, 10 20 cm. thick, 

 and traversed with plant-roots. This pressure from below, repeated 

 for years in a "rudemark," must gradually push and force the gravel 

 aside so that it lodges at last in the cracks which, while they are 

 filled with ice, form a kind of wall around each clay-prism. Thus 

 the stones are placed in the neutral territory between the small 

 centres of power, and form a boundary to each cake, the upper 

 edge of which boundary appears upon the surface while the lower 

 reaches down to the ice in the subsoil. Below the level of this ice 

 the gravel is irregularly dispersed in the clay; it is regularly arranged 

 only in the surface-layer above the ice. In the summer, when the 

 soil has thawed and the sub-surface ice melted, the water drains 

 off, and the "rudemark" dries. Everybody who has travelled in 

 Iceland during spring knows what an enormous difference there is 

 between the clayey gravel-flats in which the horses sink deep down 

 while the ice of the subsoil still hinders the draining off of the 

 water, and the same flats in summer when they are dry, so that 

 horses can gallop across them. During summer the clay-polygons be- 

 come somewhat depressed. Many of them are however slightly arched 

 during the summer also and retain for a long time a considerable 

 amount of wet in their interior. Clay which easily absorbs water 

 and expands is well known to Swedish geologists 1 who call it 

 "jaslera," and recently it has been connected with "rudemarks." 

 In the neighbourhood of Reykjavik (Melar) some well-defined "rude- 

 marks" have developed in clay soil where a water-containing layer 

 at a depth of about I 1 /!' metres rests on a thick "mohella" through 

 which water can penetrate only with difficulty, and which therefore 

 freezes in winter into a plate of sub-surface ice. Where the ground 

 consists of clayless sand no "rudemarks" are developed, nor where 

 the subsoil is so porous that water cannot accumulate and form 

 sub-surface ice proper. 



In my opinion the knolls which are of such common occur- 

 rence in the home-fields of the farmsteads (see Fig. 17) are developed 

 in a similar manner. These knolls are usually larger or smaller 

 elevations of earth which occur together in numbers: the surface- 

 layer consists of humus and plant-remains, but the interior is formed 



1 A. G. Hogbom: Om s. k. jaslera och om villkoren for dets bildning (Geol. 

 Foren. Forhandl., Stockholm, XXVII, 1905. pp. 1936). 



- E. Bergstrom: En marklig form af rutmark fran barrskogsregionen i Lapp- 

 land. Geol. Foren. Forh., XXXIV, 1912, pp. 339340. 



