James Qe'ikie — Origin of the Swedish Asar. 307 



III. — A. E. Tobnsbohm's Theory of the Origin of the Swedish 



Asar. 



By James Geikie, F.R.S.E.^ 



District Surveyor of tlie Geological Survey of Scotland. 



IN tha fourth number ^ of the Proceedings of the Geological Society 

 of Stockholm, there is a paper by Mi-. Tornebohm, of the 

 Geological Survey of Sweden, which treats of the origin of the famous 

 asar, — the same geologist has also kindly sent me some further 

 explanation of his views, — a short account of which will, no doubt, 

 be interesting to many readers of the Magazine. 



One of the most striking features of the asar, says Mr. Tornebohm, 

 is their great length — an as sometimes continuing for more than a 

 hundred English miles. Often beginning in the interior of the 

 country, an as follows some particular valley down to the low coast- 

 land, across which it passes as a well-defixied ridge out to sea. 

 In the environs of the Malar Lake, where the asar have all been 

 studied and mapped by the Geological Survey, they are found as a 

 general rule, when occurring at greater heights than 300 feet above 

 the sea-level, to be strictly confined to the valleys. At lower levels 

 they seem, on the contrary, to be tolerably independent of the 

 present configuration of the land. 



In the valleys which contain the asar, detached patches of sand 

 are sometimes found, perching high on the side slopes. These 

 patches, according to Mr. Tornebohm, are the wreck of a great 

 deposit of sand, which at one time filled the valleys from side to side. 

 While the valleys were still filled with this thick bed of sand, rivers 

 began to flow just as they now do, and cut their way down iu the sand. 

 The running water carried along with it coarse sand and gravel, and 

 deposited these on the beds of the rivers, which thus became paved 

 with coarser materials. By and by this state of things changed — 

 denudation set to work upon the whole deposit, and removed the fine 

 sand, but had not power to carry away the coarse gravel which had 

 filled up the old river courses. This gravel, therefore, remained 

 behind, and not unfrequently has protected a considerable thickness 

 of underlying sand. The annexed woodcuts, which are taken from 

 Tornebohm's paper, will further illustrate his meaning. 



Fig. 1. Fig. 2. 



Eig. 1 shows the section of a valley partly filled witli sand, s, in 

 which is cut the river-bed, paved with coarse sand and gravel, 6. 

 Fig. 2 represents the aspect of the valley after denudation has re- 

 moved the greater portion of the sand, patches of which are seen at 

 a, a. At the bottom of the valley the river gravel rests upon some 



1 Geologiska Foreningens i Stockholm Forhandlingar. Band I. 3 April, 1872. 



