262 THORODDSEN 



of the clay soil, which projects into them like a kernel and forms 

 the main part of their volume. The knolls have usually a diameter 

 of i /2 2 metres and a height of 1 /4 1 /2 metre; sometimes they are 

 somewhat smaller; or they may he larger. The form varies, but is 

 usually oblong. When the knolls are large the channels (kargatthyfi) 

 between are but narrow. The thicker soil-layer together with the 

 greensward-covering and the vegetation, are intersected by a net- 

 work of cracks similar to that in the "rudemarks:" These cracks 

 divide the ground into numerous pieces of various shapes which 

 behave, as regards the sub-surface ice and the moisture, as the 

 "rudemark." The knolls are dependent on the crack-systems of the 

 substratum. In the knolls the ascending stream of clay and humus 

 particles must be stronger than in the cakes of the rudemark as the 

 greensward, rich in humus, has a very great capillary attraction and 

 an enormous water-capacity it can absorb water to the extent of 

 50 60 /o of its volume; therefore during the evaporation from the 

 surface it absorbs water vigorously, not only from the thawing ground 

 below, but also laterally from the channels between the knolls filled 



/ 



with water during the spring thaw: in spring the greensward upon 

 the smaller knolls is as saturated with water as a sponge! Bands of 

 volcanic ashes, which were present in the ground or in the greensward 

 before knoll-formation began, become bent upwards in curves according 

 to the form of the knolls a fact which, among others, is a proof 

 of the local pressure from below in each knoll. In spring, during the 

 melting of the snow, the channels between the knolls are often half 

 filled with water which cannot escape, while the tops of the knolls are 

 dry, sometimes even very dry owing to evaporation, so that the vege- 

 tation upon them is totally different from that upon the sides of the 

 knoll. Sometimes large knolls have a kernel of ice far into the 

 summer. The formation of knolls does great damage in the peasants' 

 home-fields and great trouble is taken in levelling them, but they 

 may reappear comparatively quickly if the ground is not thoroughly 

 drained, so that the surface-water and the water from the melting 

 ice of the subsoil are immediately carried away. In this connection 

 it should be mentioned that where a snow-covering during spring 

 protects the ground for a long time against the action of frost and 

 thaw, no knolls are formed. 



Knolls of the kind described above occur in thousands also in 

 uncultivated grasslands with clayey mohella-soil, on heather moors 

 and on grass "moar", and here, also exclusively upon somewhat flat 



