GEOLOGY AND WATER RESOURCES NEAR MONTE CRISTI. 175 



Copey, but the geologic age of the coralliferous rock at several localities 

 has not yet been definitely determined. The rock at some places — for 

 instance, just south of La Barca — is definitely Miocene, but at others it 

 may be Pleistocene or even younger. 



Poorly consolidated reddish-brown argillaceous sandstone unconfoi mably 

 overlies the Miocene sediments in the lower Yaque Valley. The uncon- 

 formity is visible on the south bank of Rio Yaque at La Barca, immediately 

 upstream from the ferry landing. The consolidated Miocene beds strike 

 N. 60° E. and dip 20° SE., but the overlying soft sandstone strikes N. 25° W. 

 and dips 3° NE. The soft sandstone is probably of Miocene age. 



PLIOCENE (?) GRAVEL. 



The gravels that form the hummocks on the savannas along the foot 

 of the Cordillera Central near Dajabon may be of Pliocene age. Appar- 

 ently the gravel was originally spread over the entire plain and the hum- 

 mocks are mere remnants. The most common rocks in the gravel are 

 quartz diorite, finely crystalline black igneous rock, and quartz. At one 

 locality the gravels lie on the truncated surface of northward dipping- 

 thin-bedded sandstone and clay, probably of Miocene age. 



PLEISTOCENE (?) BEACH DEPOSITS. 



Horizontal beds of beach sand and gravel form a cliff 7 meters high on 

 the coast just east of the rope-fiber factory, about 25 kilometers northeast 

 of Monte Cristi. The recent species of mollusks listed on page 166 (sta- 

 tion 8776) were collected from beds in the upper part of the cliff. These 

 beds are not older than Pleistocene but may be younger. 



RECENT ALLUVIUM. 



The lower Yaque Valley is covered with a thin sheet of alluvium that 

 forms a yellow to brown clayey soil. The alluvium is similar to the silt that 

 is building the delta at the present time. Although the soil is fine-grained 

 it absorbs water rather readily. 



The thickness of this deposit is highly important to any one attempting 

 to obtain a supply of ground water. The deposit forms the banks of Rio 

 Yaque in its lower course, but at no place are the banks more than 3 to 6 

 meters high. Not far above the mouth of the river Miocene bedrock crops 

 out in the channel of the river at places where the alluvium is not more than 

 6 meters thick. The wells, all of which are shallow, show that the alluvium 

 is at least 6 meters thick. River deposits more than 15 meters thick will 

 probably not be found in the part of the valley that includes the largest 

 farms, but the thickness of the deposits can be definitely determined only 

 by sinking test wells. 



