78o PRINCIPLES OF STRATIGRAPHY 



5. Subaquatic, GUding-defonnation. Offshore deposits of sedi- 

 ments on a gently sloping sea or lake bottom may suffer from time 

 to time deformation of the surface layers through gliding or slip- 

 ping down the gently inclined sea floor. Such deformation has 

 been repeatedly observed in modern deposits. The best known 

 modern examples are those which affected the village of Horgen 

 on the lake of Zurich in 1875, and the village of Zug in 1887. Both 

 of these have already been described in Chapter XV, p. 658. The 

 most remarkable fact about the gliding in Zug was that it took place 

 on an average grade of 6% (3° 26'), while the larger and more 



% 



Fig. 163. Folding accompanying MiWaqualic gliding in Miocenic marl of 

 Oeningen — natural size. (After Heim.) 



pronounced movement occurred on a grade as low as 4.4% (a 

 trifle over 2° 31'). The material which thus slid into the lake 

 was brecciated and folded with overfolds, overthrusts, reversals of 

 layers, excessive strata, etc., and furnishes an excellent guide to 

 the interpretation of similar movement in the past. Among the 

 chief points in which these folds diff'er from those produced diage- 

 netically by swelling of gypsum, or by pressure of overlying masses, 

 is that they are of the nature of normal folds due to lateral com- 

 pression and so show movement in one direction only, whereas in 

 the case of the other deformations movements in several directions 

 are shown. Furthermore, the axes of the folds are thickened in 

 the gliding as in normal tectonic folds, instead of the limbs, as is 

 the case in folds due to swelling. 



