86 GUIDEBOOK OF THE WESTERN 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 yailiwad 3 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 
white valgania tuff ** about 40 feet thick and that both the lava and 
rd. The h 
sandstone dipping steeply to the northwest and is capped 
horizontal sheet of tuff and lava 
Ficurp 18.—Lava-capped hill south of Howa 
is composed of red 
ill, which is opposite mea 200, 
by a nearly 
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. ese 
*Voleanic tuff is a name applied to 
material blown out of a voleano 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. 
