50 GUIDEBOOK OF THE WESTERN UNITED STATES. 
A few miles west of Las Animas, on the north bank of the river, 
was Bents Fort,’ a noted place on the Santa Fe Trail. This fort was 
very prominent in the days when the Army of the West marched 
through in 1846, but it was demolished in 1852 because the Govern- 
ment would not purchase it at the price asked. The building was a 
very large (180 by 135 feet) one-story structure 15 feet high, with 
walls 4 feet thick, situated several miles above the junction of Purga- 
toire and Arkansas rivers, where the trail crossed Arkansas River. 
In early days the Arkansas was the northern boundary of Mexico 
westward from a point near Fort Dodge. On crossing the river the- 
caravans passed from the protection of United States troops to that 
of Mexican soldiers. 
Five miles west of Las Animas the railway approaches bluffs of 
dark Graneros shale capped by the Greenhorn limestone, which extend - 
along the south bank of the river for some distance to the west. <A 
slight westerly dip finally carries the shale below the surface of the 
alluvial filling of the valley, but cliffs or ledges of the limestone are 
almost continuous nearly to La Junta. These ledges consist of 
alternating thin beds of limestone and dark shale, a feature which is 
highly characteristic of this formation and is shown in Plate VI 
(p. 52). In an exposure 20 feet high there may be thirty alterna- 
tions of such beds. This feature indicates that, at the time of their 
deposition, there were rapid and repeated changes from muddy 
water, depositing dark clay, to clear water, from which the calcium 
carbonate, now represented by the limestone, was separated. It is 
thought that these changes were due to changes in climatic condi- 
tions. As this feature is characteristic of the Greenhorn limestone 
in a region more than 2,000 miles from north to south and 500 miles 
from east to west in ‘ec Great Plains province, it shows that the 
conditions of deposition were uniform over a wide area. 
On the north side of the river flat west of Las Animas is the edge 
of a table-land showing bluffs at intervals, in which are many small 
outcrops of the Timpas limestone, a formation about 200 fect higher 
in the geologic column than the Greenhorn. Outlying areas of this 
table-land some distance south of the tracks may be seen at intervals 
for 6 miles. A few rods east of milepost 552 a small canyon south 
'The Bent family was prominent in 
: frontier history and established succes- 
trading posts, each known as Bents 
Fort, among the Indians. 
post, up the river toward Pueblo, Colo., 
was built in 1828. The fur-trading finn 
of Bent & St. Vrain was next in size to 
the American Fur Co. Charles Bent 
was appointed Territorial governor of 
New Mexico by Gen. Kearney when 
the Army of the West took possession 
of Santa Fe. William Bent, one of the 
brothers, was a ‘‘squaw man,’ having 
married the daughter of a Cheyenne 
chief. His son, after receiving a good 
education in St. Louis, gathered about 
him a band of savage Indians and 
became so dangerous an outlaw that 
the Government offered $5,000 for his 
capture 
