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GUIDEBOOK OF THE WESTERT7 UNITED STATES. 



tain and the syncline (trough) on its east side. From a point above 

 Howard to Pleasanton Arkansas River flows in the valley eroded in 

 this syncline, and the granite on the right of the railroad lies on the 

 east side of the fault, as shown in the section. 



At Pleasanton the railroad is built on the Weber shale and sand- 

 stone near the fault, but in passing northward it diverges more and 

 more from the granite wall until it is on the Maroon sandstone 

 nearly in the middle of the trough. This sandstone makes its ap- 

 pearance a short distance above the siding of Vallie. It is very 

 conspicuous on the left, in the hill across the river, and dips about 

 70° W., or into the great syncline which lies on that side of the 

 railroad. This hill shows to good advantage not only the red Ma- 

 roon sandstone but a cap of lava, which gives some clue to the re- 

 cent geologic history of the valley. As seen from the train the lava 

 cap appears to be horizontal, but after passing it the traveler, upon 

 looking back, may see that the lava cap is underlain by a bed of 

 Avhite volcanic tuff ^® about 40 feet thick and that both the lava and 



Ftgcre is. — Lava-capped hill south of Howard. The hill, which is opposite milepost 200, 

 is composed of red sandstone dipping steeply to the northwest and is capped by a nearly 

 horizontal sheet of tuff and lava. 



the tuff slope to the west, or away from the railroad, as shown in 

 figure 18. This westward slope shows that at the time the tuff was 

 deposited and the lava was poured out upon its upper surface, the 

 deepest part of the valley lay considerably west of the channel in 

 which the river flows to-day. 



The red sandstone crops out by the side of the railroad as far as 

 milepost 200. Here it is covered by a large mass of tuff and lava 

 which descends below river level and which shows on the northeast 

 side of the valley in places to points beyond Howard. Most of the 

 high hills near Howard are capped with white volcanic tuff and 

 with a sheet of lava, which invariably slopes to the west. These 



^ Volcanic tuff is a name applied to 

 material blown out of a volcano by an 

 explosion of gas or steam. It is gen- 

 erally composed of fine particles of 

 glass but may include fragments of 



rock of different sizes. The bed of 

 tuff here may have been formed of 

 dust and ashes that settled down on 

 the ground from the atmosphere or 

 were washed into a basin or valley. 



