SOME CHAPTERS OF THE TERTIARY HISTORY OF ICELAND 3 



succession of many 5-10 cm seams separated by sandstone. I think that 

 these sediments have been formed by a sluggish river in a wide flat depression. 

 As long as there was only an occasional lava flow at extended intervals the 

 course of the river changed little. But in the end more intensive volcanism set 



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Fig. 1. Section through the western part of the Brjanslaekur sediments (looking 

 S.-SW.). On top of a lava (1) of normal magnetization, there is a mainly barren 

 lower part (2) of sediments: dark, brown, and yellow clay. A new, normally mag- 

 netized, lava (3) filled a groove (water course) which had been eroded into the 

 lower sediments. This lava was previously considered a sill, but distinct magneti- 

 zation of the clay at the lower contact, and absence of any magnetization at the 

 upper contact as well as the difference of the under- and overlying sediments tend 

 to confirm the present interpretation. The main fossiliferous sediments (4) follow. 

 The lignite forms many thin layers, which are embedded in clay and consist to 

 a considerable part of wood chips and numerous leaves, apparently carried by a 

 sluggish river and buried in its muddy bottom. A new lava (5), of normal 

 magnetization, again filled the water course and covered its surroundings. The 

 lava has a blocky (kubbaberg) structure and is partly a typical "palagonite" 

 breccia as might be found in the much younger "Moberg" formation. The river 

 dug a new shallow bed at the same place as before and deposited a new layer (6) 

 of coarser, fluviatile material. Again a lava (7), of reverse magnetization, filled 

 the bed, but a clear trace of this is now lost in the section. A coarser sediment, (8), 

 mainly non-fossiliferous, was deposited before a new and more intense volcanic 

 period, represented by basalt lavas (10) put an end to sedimentation in this 

 locality. At (9) is an intrusive basalt of reverse magnetization. 



in, there came a rapid succession of lava floods, with the result that sedimenta- 

 tion came to an end here, i.e. the river was diverted to another area or 

 perhaps it disappeared completely into the porous group of young lavas, the 

 water flowing as ground water and not as a river. 



When speaking of a lava desert, I have in mind conditions somewhat 

 similar to those of the present Odadahraun lava field. All the precipitation is 

 lost into the porous lavas and flows as ground water on a deeper impervious 

 floor until it emerges as large springs in the outskirts of the lava field. Within 



