WILLIS] GENERAL, GEOLOGIC NOTES 35 



clearly belong to different episodes of development. The older ones 

 lie inland in a zone beliind tlie younger. Tlioy arc grass-grown and 

 sink away to low mounds till they merge into the plain. Wliere the 

 drainage is ponded behind them there arc little lakes. Beneath 

 them is a layer of black earth representing the soil of the former 

 plain over which tlic dunes advanced with the advance of the sea 

 upon the coast. The younger dunes are composed of moving sand. 

 They rise directly from the existing beach, to whicli they arc obvi- 

 ously related, and form a zone a kilometer or more in width. They 

 vary from less than 10 to more than 20 meters in height, rise in a 

 long, wind-wrought curve from southeast to northwest, and on the 

 northwest side are as steep as the sand will lie. 



At the mouth of the Rio Quequen, near Necochea, tlie dunes are 12 

 to 15 meters high and advancing from the southwest have pushed 

 the river northward so that it flows through a restricted channel 

 beneath a cut bank in the loess 12 meters high. At Monte Hermoso 

 the zone of large grass-grown dunes is 3 to 5 kilometers wide, and the 

 dunes themselves attain a height of 25 meters or more above the sea. 

 Monte Hermoso is a dune 27 meters in height, the upper 20 meters 

 being sand and tlie base consisting of the Monte Hermoso formation. 

 These brief descriptions will suffice to indicate the effective work 

 which is being accomplished by the wind along this stretch of coast, 

 and to make it clear that the dunes are a strictly coastal formation 

 dependent on the proximity of the sea, formed from the beach sands, 

 and moved by the winds whose prevailing direction depends on exist- 

 ing relations of land and sea. More detailed descriptions are given 

 in the accounts of the specific points visited. 



The coquina which forms on the eastern coast of Argentina is 

 intimately related to the dunes in origin, since tlie wind is the chief 

 agent in its production. But, whereas the sand dunes are developed 

 by the more regular winds, the coquina is the product of the storms. 

 It consists chiefly of broken slielJs mingled witli sand and gravel, but 

 includes also any pebbles or other large objects, such as bones, which 

 may happen to occur with the sand. It is usually more or less 

 indurated, and this character, together with the fact that it contains 

 fragments of fossils identical with those which occur in the under- 

 lying Pampean, has led to its being considered a Tertiary formation. 

 The writer regards it as strictly equivalent in character and origin 

 with the coquina of the Florida coast, in regard to which we may 

 quote the following description:* ' 



"One of the most common of tlie marine Quaternary deposits is the 

 coquina which occurs at various points along the coast. This con- 

 sists of a mass of more or less water-worn shells cemented by calcium- 

 carbonate. The amount of cement is seldom great enough to close 



iSellards, E. H., State Geologist, in Second Annual Report of Florida State Geological Survey, 1908-09, 

 p. 153. 



