GRAND GULF GROUP. 41 
later work may require a redefinition or limiting of the term Grand Gulf. At present, 
however, the term is here used in the sense in which it was proposed by Hilgard, for a par- 
ticular series of clays and sands overlying unconformably the Vicksburg and other beds, 
and overlain in places by the Lafayette, Port Hudson, and loess. 
The Grand Gulf was first studied and named by Wailes from the type locality at Grand 
Gulf, Miss. He included in the Grand Gulf sandstone the interbedded clays, which he 
regarded as "the decomposed portions of the imperfectly formed rock." a 
At Grand Gulf, the type locality, at Mississippi Springs, and at Raymond, mentioned 
b}? - Wailes, & the sandstone occurs in ledges interbedded with white plastic clays. He fur- 
ther adds: 
At many points within the scope I have mentioned the rock crops out in the beds of the watercourses 
and upon the sides of the ridges, exhibiting, as in that in the Mississippi bluff, such an identity of char- 
acter as to induce me to characterize it wherever met with as the Grand Gulf rock. 
The sandstone of Fort Adams, or Davion rock, as it is railed by Wailes — 
is an argillo-siliceous composition of a dingy-white color in the mass, containing a small portion of 
sand, cemented together and tinged by a brownish-red metallic oxide, which pervades it in irregular 
and distorted veins and which, forming the hardest portions of the mass, gives the weathered surface 
a very rough and nodular character. 
Wailes describes the Grand Gulf in T. 5 N., R. 1 E., near Wilsons Ferry, on Homochitto 
River, as follows: 
The more compact arenaceous portion of the stratum is here about 3 feet in thickness, with about the 
same thickness, above and below, of the more argillaceous and crumbling material, which, in wet weather 
forms a very tenacious white pasty clay, rendering this a very formidable pass to the wagoners on the 
road, who have given to the ridge on which this ledge runs the name of the Devil's Backbone. 
It is evident from the aho^e quotations that it was the intention of Wailes, who first 
used the name Grand Gulf sandstone, to include in the term the clays of the same age. 
At Star, Miss, (see p. 42), clays are interbedded with the sandstone and form the prin- 
cipal part of the Grand Gulf in some of the outcrops along the railroad track. Clays are 
likewise prominent in the Grand Gulf at Raymond, By ram, Florence, and numerous other 
localities originally included in the Grand Gulf area. 
The Grand Gulf is made up of unconsolidated sands, siliceous and aluminous sandstones, 
and clays. The sandstones are usually white to light-yellowish gray in color, and but 
slightly cemented. They are composed of sharp grains of silica, with more or less clayey 
material and iron pyrites. Near the surface the oxidation of the iron pyrites in the sand- 
stone gives it a spotted rusty to yellow color and renders it undesirable for a building stone. 
In places the sandstone is little more than a mass of indurated, uncemented sand which 
can be dislodged from its original bed with a spade. In other places it is much harder, 
even approaching a quartzite. 
The sandstones interbedded with clays of this group are particularly common in the north- 
western part of the area — that is, northwest of a line drawn from Fort Adams to Raleigh; 
southeast of such a line sandstones are very rare, and the group consists of bluish to black 
clays (in places almost shales) and clayey sands. 
Throughout the Grand Gulf, particularly in the northwestern or sandstone area, crystal- 
lized gypsum and common salt, with more or less magnesian salt, are often found. "This," 
says Doctor Hilgard, c "is the case, even with many of the solid sandstones, which on expo- 
sure to the weather become covered with efflorescences of salts; and it is to this peculiarity, 
no doubt, that their want of durability is to a great extent to be attributed." 
The Grand Gulf underlies most of that part of Mississippi south of a line drawn as follows: 
Starting on Mississippi River a few miles south of Vicksburg, it runs parallel to and a mile or 
so south of the Alabama and Vicksburg Railway and passes a short distance north of Ray- 
mond. Here the boundary line between the Vicksburg and Grand Gulf bends rather abruptly 
a Wailes, Agriculture and Geology of Mississippi, 1854, p. 213. 
*> Op. cit., p. 217. 
c Geology and Agriculture of Mississippi, 1860, p. 148. 
