

CRETACEOUS AND TERTIARY—SOUTH OF WOOD MOUNTAIN. 107 
these hills are all nearly on the same plane, and this, if traced back into 
some of the larger hills and edges of the plateau, exactly coincides with 
the zone there still containing the lignite. The beds, as there exposed, 
however, seem hardly of sufficient thickness or importance. to cause an 
alteration of the strata so extensive as has taken place. It is possible, 
from the irregular nature of these deposits, that over the areas destroyed 
by combustion, the lignite has been thicker and of better quality, and that 
the fire may have been unable to extend itself into the thinner portions 
of the bed, where it is separated by clay partings and covered by such a 
great thickness of other deposits. The combustion must have taken place 
ages ago, as isolated red-topped buttes now only remain to mark what 
must have been the level of the plain at that time. (Plate VII., Fig. 1.) 
It will frequently be necessary to refer to this typical section, in 
discussing the Cretaceous and Tertiary rocks further west. 
Cretaceous and Tertiary Rocks South of Wood Mountain Plateau, 
and between the Bad Lands and White Mud River. 
262. On proceeding westward along the Line from the sections 
above described, one passes over Cretaceous beds, while the southern 
edge of the Wood Mountain water-shed plateau—which is here: co- 
extensive with the edge of the Tertiary—follows a more or less nearly 
parallel direction at a distance of ten to fifteen miles to the north. 
263. The sombre Cretaceous clays of divisision 6., may be traced 
almost continuously for a distance of about ten miles. Lower beds are, 
however, exposed, from;the ggeneral slight easterly dip of the rocks ; 
which is proved by the fact that the sombre clays, though first seen in 
the bottoms of the vallies, soon form the whole substance of the hills. 
They attain this position much more rapidly than the slight westward 
slope of the surface of the country at this place, will account for. 
About ten miles westward, near the crossing of the forty-ninth parallel 
and trail to Fort N. J. Turney, where the Wood Mountain Astronomical 
Station was established, good exposures of these rocks are again found in 
the banks of the valley of a large brook. On careful examination they 
were found to contain fossils, and specimens of Baculites compressus, 
B. ovatus, and other forms characteristic of Meek and Hayden’s 4th, or 
Fort Pierre group, were obtained. This horizon is also indicated by their 
position relatively to the Tertiary, and their lithological character. 
264. The rock is a soft clay-shale, which though fine, and regularly 
stratified, from its homogenious character hardly shows traces of its 
