6o2 PRINCIPLES OF STRATIGRAPHY 



to the Bavarian border furnishes a remarkable example of a con- 

 solidated conglomerate or pebble rock of the type described. This 

 goes by the name of Nagelfiuh, from the fact that where the rela- 

 tively small pebbles have fallen out of the matrix a depression 

 like that made by the head of a nail is seen. This rock appears 

 in several large erosion remnants within the city ; one of them, 

 the Monchsberg, rises with perpendicular walls (partly artificial) 

 and is pierced by a tunnel through which one of the city streets is 

 carried. Other remnants are the neighboring Rainberg, and the 

 more distant hill of Hellbrunn. All of these are evidently part of a 

 once continuous conglomerate bed, which has since been dismem- 

 bered by erosion. ' The cohesion of the material is such that old 

 crypts hollowed in. this rock and used in the third century as places 

 of secret worship are still in a practically unchanged condition. 



The walls show inclined bedding w'hich indicates the delta char- 

 acter of these deposits (Penck-42 :7d/-7(5(5). According to the 

 opinion of Penck, Crammer, and others, the sand and pebbles were 

 washed into a lake which occupied the Salzburg basin during 

 post-glacial time. The fact that the Nagelfluh rests upon a glacial 

 moraine, as determined by excavations and observations on natural 

 and artificial exposures, seems to indicate that this deposit is of 

 post-glacial age, though other observers have held that the age 

 of the deposit might be greater, perhaps late Tertiary. 



Nagelfluh of similar character, but probably of greater age, is 

 found in a number of localities in South Germany and elsewhere. 

 In some cases, as in Munich, it is used extensively for building 

 purposes. It is, of course, not necessarily true that all conglomer- 

 ates of this type are of non-marine origin.' though most of them 

 probably are river deposits. 



Playas or Takyrs and Salinas. In the low, flat-bottomed 

 depressions of undrained desert basins the rivers at times of flood 

 will spread out into extensive shallow lakes of temporary existence. 

 In the Great Basin region of western North America one such 

 temporary lake reaches a length of about 100 miles by a breadth of 

 12 to 15 miles, but with the water scarcely more than a few inches 

 deep. Here the fine silt of the river is deposited, gradually subsid- 

 ing as the shallow lake evaporates. After complete evaporation a 

 smooth, hard-baked surface remains, marked by sun-cracks and 

 the tracks of animals which visited the spot before complete harden- 

 ing of the mud had occurred. Raindrop impressions likewise re- 

 main on such a surface. In structure the material is beautifully 

 and finely stratified, as may be seen on the sides of the sun-crack 

 rifts. This constitutes the playa of the American deserts (■Mexico 



