PHYSICAL GEOGRAPHY 265 



knolls are formed of the rootlets, leaf-rosettes, etc. of various plants. 

 Sometimes knolls are formed of organic material in places where 

 a small spot is constantly manured, e. g. the so-called "bird-knolls" 

 on higher levels, where bird of prey are in the habit of alighting, 

 and the characteristic Icelandic "dog-knolls" (hundapufur) along the 

 roads where the dogs are wont to stop, which almost all travellers 

 in Iceland have for companions. 



