126 



/. A. UDDEN 



sandstone. The same horizon is exposed in the bottom of Bosque 

 River, at Clifton, 155 miles northeast from the locality just men- 

 tioned, and again some 6 miles north of Clifton in the same beds in 

 the same stream. Some layers of limestone here show ripple 

 marks that measure 4 feet across, near Clifton (see Fig. 4), and from 

 2 to 3 feet across at the northernmost locality (Fig. 5). The lime- 



Fig. 3. — Ripple marks in thin-bedded sandy limestone in the bottom of Guada- 

 lupe River, about 17 miles southwest of Kerrville, Texas. 



stone layers here are compact and quite pure in composition, but 

 are interbedded with marly shales. 



Perhaps it may be permitted to submit some general remarks 

 anent the phenomena of ripple marks. They shall be brief. Ripple 

 marks must be due to rhythmic variations in currents in the medium 

 of sedimentation. They are in this respect kin to wavelike etchings, 

 known to be caused by rhythmic movements of corrading currents. 

 Perfectly symmetric ripple marks are probably the result of to-and- 

 fro movements of equal extent in both directions, when these move- 

 ments are such that the velocity of the motion happens to be 



