TERTIARY AND RECENT GLACIATION OF AN 

 ICELANDIC VALLEY 



HENRY G. FERGUSON 



During the summer of 1905 I had the opportunity of accompany- 

 ing Dr. Helgi Pjetursson, the well-known Icelandic geologist, on his 

 trip through the west and north of Iceland. Our primary purpose was 

 the study of the sedimentary beds included between the lava flows 

 of the Tertiary basalt plateau of the island. Dr. Pjetursson has 

 made some remarkable discoveries concerning these sedimentary 

 beds, the most interesting of which is the finding of glacial deposits — 

 apparently indurated ground moraines — between the older lava 

 flows. Dr. Pjetursson has already described these formations,^ but 

 as they are little known to American geologists, I propose to describe 

 one of the areas where these beds are best developed, the Botnsdalr 

 valley at the head of the Hvalfjord, about 30 miles northeast of 

 Reykjavik. 



The basalts forming the walls of the valley dip eastward at an 

 angle from 5° to 10°, this eastward dip being constant for the whole 

 western part of the island, except, occasionally near the west coast, 

 where the boundary faults of the island cause a westerly dip. These 

 tilted basalts are called "regional" by Dr. Pjetursson, as being com- 

 mon to northern Scotland, the Orkneys, the Shetlands, the Faroes, 

 Iceland, and western Greenland; in distinction from the "insular" 

 basaltic lavas, peculiar to Iceland. These "regional" basalts are 

 roughly divided into a lower or black group, which contains the " Sur- 

 turbrandur" Hgnite and ash deposits, with Miocene plant remains; 

 and the gray group, which, like the gray group of Ben More^ in the 

 Mull plateau, is the upper group in the Icelandic plateau. The 

 latter is equally deformed, but is separated from the black by an 

 erosional unconformity. The basalts of the Botnsdalr region seem 

 to belong high up in the black group. 



I Helgi Pjetursson, "Om nogle glaciale og interglaciale Vulkaner paa Island," Det 



kgl. Danske Videnskahernes Selskabs Forhandlinger, No. 4, 1904. 



