140 



G. K. GILBERT RIPPLE-MARKS AND CROSS-BEDDING 



On part of the floor of the Whitmore quarry at Lockport the ripple 

 pattern seems to have been a reticulation, including oval troughs, and 

 deposition associated with a current parallel to the trough axes produced 

 the spoon-shaped la3'ers previously referred to. Such reticulated pat- 

 terns, supposed to be caused by two systems of waves, either simulta- 

 neous or successive, are often observed on a smaller scale. 



When 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 associ- 

 ated with the old ridges is truncated 

 so that the new lamination is uncon- 

 formable (see figure 5). Where the 

 ripple pattern is small several un- 

 conformities of this chraacter may 

 appear in a hand specimen, and 

 where they occur on a large scale in the quarries of the Medina sand- 

 stone they make a complexity of structur-e greatl}^ impairing the value 

 of the stone for architectural purposes. 



It is proper to add that I do not regard sand-rippling as a general or 

 even frequent cause of the cross-lamination observed in rocks, except on 

 a small scale. In some of the quarries of the Medina sandstones there 

 is cross-lamination without any indication of associated rippling, and I 

 am satisfied from personal observation that the conspicuous cross-lami- 

 nation exhibited in certain quarries of the Berea sandstone at Elyria and 

 Amherst, Ohio, as well as the superlative cross-bedding of the Juratrias 

 sandstones of Utah, Arizona, and New Mexico, were formed in some 

 other way. 



Figure 5. — Complex Cross-bedding and Unconform- 

 ities associated with Ripples. 



A change in the direction of the water waves 

 superposes the eomponnd cross-bedding b-b on 

 the compound eross-bedding a-a. 



