22 GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
son was led to give a very correct explanation of the origin and mode of formation of the nu- 
merous round, flat-topped mounds of that region.! 
Upraised sandstone strata.—It should be remarked, before passing to a , consideration of the 
portions of the line to the westward of this region, that Captain Marcy in his exploration of Red 
river passed within twenty-five or thirty miles iof the south bank of the Canadian, and at one point 
crossed over toit. Dr. Shumard, who aecompanied him, made some observations on the strata 
of the region, which will be interesting in this place, and tend to throw additional light upon 
the geology of the line. At a point several miles south of the trail, between Camps 36 and 38, 
Dr. Shumard found an outcrop of greyish yellow sandstone, presenting a dip of forty degrees 
to the west. Again, on the 9th of June, near Sweet Water creek, an outcrop of finely lami- 
nated red, ferruginous sandstone, presenting an irregular dip to the northeast of about thirty 
degrees, was found. It wasalso passed during the next day's march and presented evidence of 
violent disturbances ; the strata being nearly perpendicular in some places. On Red river, about 
twenty miles east, horizontal strata of red and blue clay, containing gypsum, were seen, and they 
formed long ranges of bluffs. On the banks of Suydam creek there were similar bluffs, but 
without the gypsum. From these facts I conclude that the upraised sandstone is older than 
these gypsiferous strata, and that it was uplifted prior to their deposition. "The upraised beds 
are probably similar in composition and age to the upraised carboniferous sandstones occurring 
east of Delaware mountains in the Sans Bois and other ranges. 
Loess.—Another and a very interesting formation was found in thatregion by Dr. Shumard, 
on the banks of a creek just south of Dry river, and between Suydam and Buffalo creeks. This 
deposite consists of a grey calcareous loam, filled with shells characteristic of the loess forma- 
tion— Helix plebium, Succinea elongata, dec. A similar deposite was found on Red river, twenty- 
six miles above Fort Washita. At this place it is ‘‘ about twenty-five feet thick, and consists of 
an ash-colored calcareous loam, which, on inspection, was found to contain terrestrial and fluviatile 
shells of the genera Lymnea, Physa, Planorbis, Pupa, and Helix, the whole resembling species 
which we have observed at New Harmony, Indiana, and elsewhere in the Mississippi valley.''? 
It is this formation which rises in bluffs along the Mississippi river, and which was deposited by 
the river when the land was slightly lower than at present ; and now, owing to its uplift, the 
river has cut a path downward and carried away a great part of the deposite, but has left here 
and there a ** bluff’’ or terrace to mark its former channel. These bluffs at Natchez attain an 
elevation of about 200 feet above the more newly deposited alluvium, and in these and at other 
places the land and fresh-water shells are found to be specifically identical with those in the 
adjoining swamps. Sir Charles Lyell, during his visit to this country, first recognised the simi- 
larity of these deposites to the loess of the Rhine.’ 
It is therefore probable that the region of the upper portion of the Red river has been sub- 
jected to changes of level similar to and synchronous with those which have affected the valley 
of the Mississippi. We should consequently look for similar evidences of disturbance along the 
Canadian, especially along that portion of it near Loess creek. It is very probable that such 
deposites exist at various points, and they may be very extensive and cover broad areas in the 
low valleys bordering the lower parts of the river, especially near its junction with the 
Arkansas. 
Dr. Shumard also observed the occurrence of considerable quantities of ** drift’? scattered 
over the region, which must be similar to that met with along the line. This ** drift’’ deposite 
will be considered after the description of the Llano Estacado has been given. 
l Llano Estacado, (the Staked Plain).—At Camp No. 45, on the banks of the Arroyo 
Amarillo, the ascent of the bluffs of the Llano was commenced, and the surface of the plateau 
was followed for about twenty-seven miles, with the éxito ptio of the crossing of Encampment 
! Report of the route from Fort Smith to Santa Fé, p. 10. 
? Shumard in Marcy's Red River report, p. 182. 
? Lyell's Second Visit to the United States, vol. ii, chap. xxxiv. 
