508 H. E. Gregory — Fluviatile and Marine Gravel. 



velop. If the climate be arid, streams of fluctuating discharge 

 score the slopes and construct terraces among which dunes may 

 be found as illustrated by the coast of Peru. The stranded 

 gravels may be destroyed by a readvance of the sea or dis- 

 sected by streams or preserved by burial under delta or river 

 deposits. 



When preserved in the sedimentary record the evidence of a 

 receding sea is a widespread stratum composed of lenses of 

 conglomerate usually much less than 100 feet in thickness 

 underlying, with marked uniformity, fluviatile deposits fre- 

 quently coarser in texture. Within the conglomerate, lacustrine 

 shales and peat or coal and evidences of seolian action may 

 be preserved. 



This analysis of gravel deposition conditioned by coastal up- 

 warp is readily checked by an examination of existing coastal 

 plains, of which Patagonia is a typical example. As interpreted 

 by Hatcher,* the coastal plain of Patagonia extends from the 

 base of the Andes to the sea and is covered by the "Shingle 

 Formation," with an average thickness of 30 feet. The marine 

 gravels consist throughout of a heterogeneous mass of water- 

 worn stones with but a slight admixture of sand and clay and 

 unconformably overlie the Cape Fair weather beds of marine 

 origin. The ancient beaches and sea cliffs are in places still 

 preserved. 



Conclusion. 



There are doubtless more gravel and bowlders strewn over 

 the surface at present than at any time in the earth's history. 

 Youthful and mature landscapes are common, old age forms 

 are rare. 'Vigorous waves and rivers are supplying gravel in 

 maximum amounts, and the extensive coarse deposits from Pleis- 

 tocene and recent glaciers are still in place. Of the gravel now 

 in process of deposition by far the greater amount is furnished 

 by the continental agents, rivers and glaciers. Marine erosion 

 yields relatively little. The proportional efficiency of these 

 agencies was doubtless not essentially different at any previous 

 geological period. Thickness and extent and position of gravels 

 and of conglomerate are therefore factors of much significance 

 not only for the reading of the life history of existing rivers and 

 coast lines but also for the interpretative description of ancient 

 landscapes. 



Yale University, New Haven, Conn., 

 December, 1914. 



* Princeton Univ. Exped. to Patagonia, vol. i, p. 221, 1903. 



