Fluviatile and Marine Gravels. 493 



at the upper end and terminating in an abrupt or in a gentle 

 slope downstream. These lenses are roughly parallel with the 

 axis of the immediate valley and are separated by bands of 

 sand or of silt in places underlying shallow pools of water. In 

 addition to the lenticular masses mentioned there is another 

 set of bars which traverse the channel obliquely joining the 

 inner banks of two successive curves. In several places these 

 oblique bars are the only gravel deposits present and the 

 dwindling stream was observed to cross these bars from one 

 pool to another. The pattern woven by these sets of bars is a 

 net stretched in the direction of stream flow. In the meshes 

 of the net are areas of sands and muds of various textures. 

 Between floods the net appears to migrate down stream with- 

 out substantially altering its pattern. By wading in the 

 stream at flood stage it was found that the position of pool and 

 bar with references to each other was unchanged. One char- 

 acteristic feature, which I have not observed in sediments of 

 non-fluviatile origin, is the presence of lines of detached pebbles 

 embedded in the sand and arranged like widely-spaced beads 

 on a string. In places an isolated pebble is found resting on 

 sand a foot or more from its nearest companion. In streams 

 with widely fluctuating discharge the pattern of gravel lenses 

 is believed to be maintained and if carried below base level 

 the gravel may be preserved throughout a cycle, and is likely 

 to become consolidated into conglomerate. If stranded above 

 base level these deposits are subject to mutilation or destruc- 

 tion in response to modification of grade induced by tectonic 

 or climatic changes, as is abundantly illustrated *by streams of 

 Plateau Province. 



When preserved in the sedimentary record inland stream 

 conglomerate deposited during a single cycle lies conformably 

 or unconformable on older deposits. The conglomerate, pre- 

 vailingly crossbedded, is arranged in long, roughly parallel 

 bands or courses, in the direction of dip or displayed as strands 

 in an interlacing pattern as shown by Johnson.* Along the 

 strike lenses and belts of conglomerate irregularly alternate 

 with lenses of sandstone. Lenticular deposits of shale and 

 even of peat or lignite may be present since quiet water bodies 

 of small extent are normal features in regions of uneven stream 

 deposition particularly where winds are active. Lenses of con- 

 glomerate in roughly parallel or interlaced order a few tens, 

 or at most a few hundreds, of feet wide, and tens or rarely a 

 few hundreds of feet thick, suggest fluviatile origin. Such 

 deposits as shown in section exhibit no regular gradation in 

 coarseness. If the streams are provided with distributaries 

 in the zone of gravel deposition the width of the combined 



*U. S. Geol. Survey, 21st Ann. Eept., 1899-1900. 



