1 1 2 THE ENVIRONMENT 



State, and mainly present as limonite, Fe^Os. n H2O. This is the 

 common iron mineral formed during superficial, oxidative weather- 

 ing. Red beds were formed mainly in ancient deserts and are com- 

 parable to similar deposits now forming in existing deserts. 



We know typical red beds from the 'normal', later geologic history, 

 such as the 'Old Red' of Devonian age and the 'New Red' of 

 Triassic age of Britain, which are some 400 my and 200 my old 

 respectively. There are comparable red beds in the Silurian of North 

 America, some 450 my old, whilst during the Permian system, around 

 250 my ago, red beds developed quite extensively in many places 

 on earth. 



The knowledge gained from present desert formations and from the 

 red beds of the later part of the geologic history can be extrapolated 

 to earlier times. Surprisingly enough at first, but on the other hand 

 quite consistent with our ideas about an early anoxygenic atmos- 

 phere, we find that there are no really old red beds. 



There are pre-Cambrian deposits of red beds. But, just as with 

 most pre-Cambrian fossils, these belong to late pre-Cambrian times. 

 The Torridonian sandstones of Scotland and the Jotnian sandstones 

 of Fennoskandinavia form a case in point. Both are pre-Cambrian 

 in age, but both belong to that late pre-Cambrian orogenetic cycle, 

 to which also the lower Paleozoic belongs. They belong to the same 

 sequence, and although they seem to be quite different from the 

 Cambrian by their lack of fossils, they are not, in fact, much older. 



Neither the Torridonian sandstones, nor the Jotnian sandstones, in 

 which the Dala sandstones form perfect red beds, are well dated. 

 But accepting an age of 600 my for the base of the Cambrian, we 

 can safely ascertain the age of the pre-Cambrian red beds to be less 

 than 1000 my; less than one billion years. 



It is quite possible that older red beds occur, and it even ought to 

 be an object of further research into the geological aspects of the 

 origin of life to try to arrive at a dating of the oldest red beds. 

 However, up till now dates older than 1 billion years for red beds 

 do not seem very reliable. I will give two examples, both of the 

 Canadian old shield. Around lake Athabasca, in western Canada, 

 much older red beds have been described. This results, however, 

 from confusion of the much older, well dated, rocks of the basement 

 with the much younger red beds, that are even of paleozoic age 



