STRUCTURE OF DELTAS 613 



Examination of deltas permanently or temporarily laid dry 

 shows that the structure is by no means a uniform or simple one 

 throughout. Thus the delta of the Dundelbach, laid open to obser- 

 vation by the partial drying of the Lake of Lungern into which it 

 was built, showed striking variation. Near its head it consists of 

 beds of coarse and fine gravel, sloping at an angle of about 35 

 degrees. Large, flat rock fragments rest with their surfaces on 

 the inclined gravel layers, and a bed of compressed bituminous 

 woods and leaves, six inches in thickness, is interbedded with these 

 gravels in one part of the section. In general, the successive in- 

 clined layers are only a few inches thick ; and this thickness does 

 not increase toward the lower end in the coarser layers. In the 

 fine mud layers, however, there is an increase in thickness down- 

 ward before the layers bend over horizontally at the bottom. These 

 finer textured layers rest gently against the steeper, coarser ones, 

 filling especially the angle between the steeper layers and the flat 

 lake bottom. 



The numerous (20 or more) well borings made into the con- 

 fluent deltas of the Po, Etch, and Brenta, in the region about 

 Venice, have revealed the fact that the structure of the delta is an 

 extremely heterogeneous one. While the beds are, in general, 

 horizontal, with only minor undulations, the succession is scarcely 

 the same in any two of the bore holes. This proves that the 

 beds of the delta form a succession of lenticular masses, of very 

 limited extent. Only two sandy layers, carrying water, .have 

 proved in any way. constant ; all other layers quickly wedge out 

 laterally. (See the combination of profiles given by Credner- 

 14: pi. I, Fig. p.) The successive layers comprise brown clays 

 alternating with yellowish sands with lignites and occasionally 

 layers containing marine organisms. The series includes several 

 beds with molluscan remains. Marine molluscs, especially Cardi- 

 acea, abound in the higher fossiliferous layers, while in the deeper 

 beds only occurs the intermingling with these of fresh-water types. 

 In some wells lignites with associated land snails, such as Succinea, 

 Pupa, Helix, etc., were found. Vegetable material occurs at four suc- 

 cessive horizons in the Po delta down to a depth of 100 meters. The 

 material is the same as that now forming marshlands on the coast 

 of the Adriatic. This and the occurrence of the land snails in the 

 lower beds would suggest subsidence since delta-building began 

 here. The borings near Venice show that about one-third of the 

 material making up the upper sixty meters of the delta ground con- 

 sists of lignite and peat. 



Quite a dififerent picture from this is presented by the mud 



