7o6 PRINCIPLES OF STRATIGRAPHY 



profile of the troughs along certain tangents. The tangent planes 

 are often nearly horizontal, in which case the cross-bedding would 

 approach in appearance the torrential tyj^e. The absence of divid- 

 ing strata would, however, readily distinguish it. Cross-bedding of 

 this type has been observed by Gilbert in the Medina sandstone of 

 western New York, and referred to wave work. 



"When the waves from a new direction act on a surface already 

 rippled, they produce a new pattern, which at first combines with 

 the old one, but eventually obliterates it. The troughs of the new 

 pattern are formed in part by excavation from ridges of the old, and 

 the lamination associated with the old ridges is truncated, so that 

 the new lamination is unconformable." (12: jyo. Fig. 5.) Sev- 

 eral such unconformities may succeed each other, and Gilbert 

 holds that the irregular cross-bedding of the Medina sandstone, 

 referred to above as perhaps of eolian origin, was produced in this 

 manner. It may be doubted, however, if ripples of a sufficient mag- 

 nitude to produce such a structure are ever produced under water, 

 and still more if when produced they are accompanied by such 

 rapid deposition as the case would seem to require. For, as Gil- 

 bert has pointed out. the formation of large ripples requires great 

 waves, and therefore broad and deep water bodies, and in such 

 deposition of sands is not extensive. On the whole, the structure de- 

 scribed conforms much more nearly to the observed structure of 

 anemoclastic deposits. It may also be questioned if the ripple cross- 

 bedding on a small scale may not with equal or perhaps greater 

 facility be produced by the wind alone. It would seem that shift- 

 ing wind ripples, which are sand dunes on a small scale, would pro- 

 duce the same structure that shifting sand dunes produce on a 

 larger scale. 



Strata which show the irregular type of cross-bedding must be 

 carefully* scrutinized for other evidence of eolian activity as well 

 as for evidence of marine or fluviatile origin. The occurrence in 

 the rock of scattered marine organisms is no conclusive evidence 

 of the marine origin of the formation, unless it can be shown that 

 the organisms in question lived where found or at least were car- 

 ried there by currents of water and not by wind. 



3. Beach Cusps. (Johnson-i6.) Beach cusps are triangular 

 ridges extending across the beach generally at right angles to the 

 shore front. When most typically developed the beach cusp has the 

 form of an isosceles triangle with its base parallel to the beach, but 

 at its upper edge, and its apex near the water. The cusp may be 

 broad, approaching in form an equilateral triangle, but more gen- 

 erally it is long, narrow and extremely acute, the sides sometimes 



