JACKSON FORMATION. 35 
The dip to the west was estimated by Mr. Langdon to be 6 to 7 feet per mile. 
From the above it seems that there is a great irregularity in the dip of the Jackson forma- 
tion. From Jackson southward the dip to the south is very great, but farther north it 
gradually flattens, and probably becomes a slight dip to the north. The irregularities in the 
dip of the strata in Mississippi may be due to the same cause as that which in Alabama pro- 
duced the lower Peach Tree anticline, Hatchetigbee anticline, and Bethel fault. a 
The stratigraphic position of the Jackson is between the Claiborne and Vicksburg, as 
pointed out by Conrad, Hilgard, Smith, and others. Its area is more extensive than that of 
either the Claiborne or Vicksburg, occupying a stretch of country which is narrowest in the 
east and gradually widens to the west. The increasing width to the west is explained by the 
fact that in the east the dip to the south is much greater than in the west, where the country 
becomes more level and the dip in some places almost horizontal. 
The Jackson area can usually be traced across the country by the rich calcareous prairie 
soils. Owing to the absence of deep-well records over the area, it has been impossible to 
determine the thickness of the formation. Estimates have been made from the width of 
the outcrop, but the irregularities in the dip of the strata in the vicinity of Jackson have 
rendered the results unreliable. 
In Alabama the thickness of the St. Stephen, which includes the Vicksburg and Jackson, 
is estimated at 350 feet. The Vicksburg in Mississippi is less than 110 feet thick, so that if 
the combined thicknesses in Mississippi are the same as in Alabama we have 240 feet for the 
Jackson. It is doubtless much thicker, however. The bluff at Yazoo City shows 180 feet of 
Jackson clays in one continuous section, and the formation outcrops along the bluff for 12 
miles to the north and about 30 miles to the south. With a thickness of 180 feet at Yazoo 
City, near the northern limit of the area, and with a very slight southward dip, it is more 
than probable that the entire thickness is at least 350 or 450 feet. 
The type locality of this formation, as above mentioned, is at the city of Jackson. This has 
been the classic collecting ground for the paleontologist, and hundreds of specimens have 
been found here. The beds in which the well-preserved fossils occur are found at the wagon 
bridge across Pearl River, on Town Creek, near its mouth, and along a deep ravine about 1 
mile north of the old capitol. At none of these localities are the strata horizontally bedded, 
nor do they preserve their lithologic unity so that any one bed can be traced for any great 
distance. The gray or greenish marl, replete with fossils, may in 100 feet change to a black 
lignitiferous clay. Jackson fossils have also been collected at Moodys Branch and McNutt 
Hill. 
The high bluff skirting the east rim of the Yazoo bottom is in most places very steep. The 
top of the bluff is covered with a much younger formation, the loess, which will be described 
later (see pp. 48-49). Underneath this loess capping the Jackson may be seen for a distance 
of about 35 to 40 miles south of Free Run, in northern Yazoo County. Many of the streams 
have cut through the loess and have exposed the Jackson for some distance back from the west- 
ern edge of the bluff. At Yazoo City and for several miles north and south this bluff affords 
excellent sections of a gray to bluish clay marl bearing a typical Jackson fauna. The height 
of the bluff above the river is 280 feet. The lower 180 feet are composed of gray calcareous 
clay containing crystals of gypsum, Zeuglodon bones, and other Jackson fauna. Above this 
are 10 to 12 feet of Lafayette sands and gravel, and this in turn is overlain by about 100 feet 
of loess. 
The "bald prairies," underlain by the gray calcareous clays bearing the bones of Zeuglo- 
don, can be traced across the counties of Madison, Scott, northern Smith, Jasper, southern 
Clarke, and northern Wayne, into Alabama. 
The country along Chickasawhay River and some of its tributaries affords better oppor- 
tunity for collecting Jackson fossils than the central part of the area. Along the Chickasaw- 
hay, in sec. 21, T. 1 N., R. 16 E., Clarke County, is a bluff 30 feet high of calcareous clays, 
from which Hilgard b collected many Jackson fossils. He mentions, among other localities 
a Coastal Plain of Alabama, pp. 207-211. 
b Geology and Agriculture of Mississippi, 1860, p. 134. 
