7o8 PRINCIPLES OF STRATIGRAPHY 



lo to 30 feet, and their height from 6 inches to 3 feet. Fairchild 

 has found crests 80 feet apart, but it is not certain that all such 

 structures are referable to cusps. 



4. Wave Marks. On shallow coasts the advancing waves slide 

 up onto the shore after breaking, forming the "swash." After the 

 retreat of the wave, its furthermost advance is found to be marked 

 by a fine, wavy line, corresponding in outline to that of the water's 

 edge, and composed of fine particles of mica, fragments of seaweed, 

 fine sand grains and other matter light enough to be carried along 

 by the water. Numerous wave lines of this character may generally 

 be seen on a shore of the type noted. In exceptional cases, as in 

 the Medina sandstone of New York, these are finely preserved after 

 the consolidation of the rock, appearing often as perfect as on the 

 unconsolidated beach. (Fairchild-7.) 



5. Rill Marks. The water running oflf after each swash, or on 

 the retreat of the tide, frequently cuts rills into the surface of the 

 beach. These rills represent a river system in miniature, and gen- 

 erally consist of a number of small, quickly widening channels, 

 which join a trunk channel at a very oblique angle, and which are 

 in turn joined by other branches at an oblique angle. 



A ditTerent type of rill marks is found where small streams de- 

 bouch upon a flat, sandy or clayey plain. Here the waters of the 

 stream Will divide into innumerable fingers and fingerlets, the re- 

 verse of the river-system type. Thus before the water sinks into 

 the ground or runs ofif, a series of channels, branching more and 

 more in their lower courses, have been produced. These channels 

 are reproductions on a small scale of the large channels spreading 

 over the subaerial deltas at the debouchure of desert streams. 



After the dying out of the streamlets which produced the rills, 

 the conditions are generally favorable for the preservation of these 

 in the hardening mud, and through covering by wind-drifted sand or 

 flood deposits. On the shore conditions for the preservation of rill 

 marks are less favorable, since the succeeding wave will generally 

 destroy the marks left by the run-ofif preceding. Occasionally, how- 

 ever, such a preservation may occur. In either case, the filling mud 

 or sand will on hardening show the relief of the original rill, re- 

 producing the minutest channel as a raised ridge. These relief 

 structures greatly resemble branching stems of plants and have 

 sometimes been described as such. (Rogers, Lesquereux, New- 

 berry.) 



Water flowing down the beach is often checked locally by peb- 

 bles or shells lying partly buried on the beach. In such cases the 

 water flowing down on either side of the obstruction will excavate 



