GYPSUM FORMATION—TABLE-MOUNDS—ROCK MARY. 
17 
II. FROM DELAWARE MOUNTAIN TO THE ANTELOPE HILLS. 
After passing Delaware mount, the hills of sandstone and shales, which, before reaching the 
mountains, were always in view underfoot or at the south, are no longer visible ; and the 
country becomes an undulating plain, covered with grass. Here the vast deposites of red sand¬ 
stones, red clays, and marls containing beds of gypsum, commence and extend without inter¬ 
ruption or change over the entire section under consideration, and beyond to the mountains of 
Santa Fe. 
Mr. Marcou considers this formation to he the equivalent of the formation known as the new-red 
sandstone, or Trias, and has so named and described it in his notes and Resume. The same 
formation was traversed by Captain Marcy in his exploration of Bed river ; and Dr. Hitchcock, 
in describing his specimens and reporting on the geology of his route, does not assign its age, 
but calls it the “ gypsum formation and I shall frequently use this name in the following 
descriptions. 
As the mineral characters and appearance of the formation, as it was seen during each day’s 
march, are very minutely given by Mr. Marcou in his notes, I refer to them for the details ; 
but present here the description of some of the most interesting points along the line where 
the characters of the formation are well represented. 
Immediately after leaving Delaware mountain, Mr. Marcou mentions passing hard, blue 
clays and fragments of gypsum. Between Camps No. 20 and No. 21, blue clays, overlying 
gypsum, and a coarse-grained and feldspathic sandstone, were traversed ; and between Camps 
21 R,nd 22, blue clays overlying red sandy clay, and white shales, were seen. These strata were 
horizontal. Between Camps 22 and 23, red clays were found, overlaid by a sandy, red brecciated 
limestone, passing into a reddish grey sandstone ; the strata dipping to the west-southwest at 
an angle of from five to ten degrees. At Camp 23, red sandstones, alternating with grey, were 
found. Fragments of gypsum became abundant beyond Camp 26. 
Mounds and columns of sandstone .—Denudation mountain, at Camp 29, is formed of horizontal 
strata, and results from the washing away of the surrounding country. 
Rock Mary is of similar origin. This a well-known land-mark, and is a striking and inter¬ 
esting object to the traveller on the monotonous plains. Rock Mary owes its name to Lieutenant 
Simpson, who describes it in his report of 1850. He remarks : “ The rock is situated solitarily 
in a prairie plain ; its height some sixty feet, its base some two hundred feet in diameter. 
In form it is like a pound-cake well puffed up, and partially broken at its centre. Two turret¬ 
like projections are seen protruding from its top. In substance it is an unstratified sandstone, 
of a red color. In surface it is spirally corrugated, and in places somewhat gnarled, owing to 
the degradation caused by atmospheric and aqueous agents. During the latter part of our 
journey a number of these hills have been seen. Besides the one just mentioned, I ascended 
two others, one of them being about one hundred feet high. These two last have calcareous 
rock in association with red sandstone. On one of these, of abutment shape, I noticed amor¬ 
phous red sandstone to be the basis rock ; next in the ascending series, five feet of red shale, 
purely schistose ; next, three feet of arenaceous limestone ; and lastly, crowning the other forma¬ 
tions, calcareous rock of massive character, containing crystals of carbonate of lime.” 1 
Mr. Marcou mentions eight or ten great cones truncated at the summit, and from 60 to 100 
feet high. The strata are horizontal, and thick strata of red sandstone predominate, with 
alternations of red shales more argillaceous and separating in thin leaves. Two or three beds 
of a siliceous limestone occur at the upper part, of a whitish grey color, very hard, and which 
has preserved the lower strata from denudation, forming natural mounds. This is a translation 
of Mr. Marcou’s description. 
1 Simpson’s report of the route from Fort Smith to Santa F£, p. 8. 
3 t 
