EARLIER TERTIART (EOCENE AND OLIGOCENE). 733 



" The formation is composed mainly of a fine-grained clay that is very dark gray or in places 

 almost black when wet, but which becomes a light gray on drying. It is familiarly known in 

 the region as soapstone. Interbedded with this clay are sometimes found, especially in the 

 lower part of the formation, beds of fine micaceous silty sands, which are usually indurated to 

 soft sandstones. The lower part of the formation also contains interbedded with the gray clay 

 and micaceous sand beds of greensand that may contain enough calcareous matter to cement 

 certain layers into pure limestone. The calcareous matter has^ doubtless been derived from 

 marine shells, the hollow impressions of which are abundant in some of the more calcareous 

 beds. Such beds have been found near the base of the Portera Creek formation at intervals 

 from a point just east of Middleton, Tenn., nearly to Paducah, Ky." 



The thickness of the formation is about 175 feet, and Glenn states that it "outcrops imme- 

 diately west of Ripley in a belt about 8 miles wide in southern Tennessee, but averages only 

 about 4 mUes in width across the State. In Kentucky it widens out again, reaching 10 or 12 

 miles in northern Calloway County. The outcrop narrows much as it curves westward beyond 

 Paducah and is concealed by the alluvial deposits of Ohio River before crossing the Illinois. 

 In Illinois it is known to outcrop only along the bank of the Ohio, at Caledonia Landing, and 

 for some distance to the north toward Grand Chain. The exposures are for the most part poor, 

 however, and its identification is made partly by a few indistinct fossil casts but mainly by the 

 presence of greensand, which is absent from the Ripley below and the Lagrange above but 

 which is found in the lower part of the Porters Creek. Farther west, across southern Illinois, 

 its outcrop is obscured by either the Lafayette gravels and the loess or by the alluvial deposits 

 of the Cache and Mississippi River bottoms." 



' Shepard'''* states that the Porters Creek formation "has been identified in Missouri wells 

 at but one point. It was reached in the Morehouse well at a depth of 248 feet, where it con- 

 sisted of 197 feet of bluish gumbo." 



Wilcox group. *"*> ^"^ — This group takes its name from Wilcox County, Ala., where imusually 

 good exposures of it are recorded. In that State it forms the most massive of the Eocene 

 groups, having a thickness of probably not less than 900 feet. It also presents a great variety 

 in lithologic character and in fossil contents. In general terms the Wilcox strata are cross- 

 bedded sands, thin-bedded or laminated sands, laminated clays, and clayey sands, with beds 

 of hgnite and lignitic matter. With these are fomid, interbedded at several horizons, strata 

 containing marine and estuarine fossils. The fossil-bearing beds form the basis for the separa- 

 tion of this group into four formations, which Smith '*°* describes as follows: 



"The lowest formation of the Wilcox group, the Nanafaha, overlies the Naheola formation 

 of the Midway and maintains a tolerably uniform thickness of about 200 feet entirely across 

 the State. These beds are mostly sandy but contain great numbers of the shells of a small 

 oyster, Gryphsea thirsse. Near Alabama River and for a short distance to the east a gray sili- 

 ceous clay with a tendency to indurate into a tolerably firm rock resembling very closely some 

 of the strata of the Tallahatta buhrstone of the Claiborne group, * * * is a characteristic 

 feature of the whole section. At the base of the oyster-shell beds there are, at certain localities, 

 other fossiliferous beds containing a great variety of forms. 



"At the bottom of the Nanafalia formation there is a bed of lignite, 5 to 7 feet thick, which 

 m^y be traced across the country from Tombigbee River into Pike County, where it is well 

 exposed near Glenwood station, not far from Troy. 



"The Tuscahoma formation, which overlies the Nanafalia, is about 140 feet thick and 

 consists mainly of gray and yellow cross-bedded sands and sandy clays, generally poor in fos- 

 sils except at one horizon, which is typically exposed at the locality from which the name is 

 taken and at Gregg's and Bell's landings on Alabama River. 



"Above the Tuscahoma is the Bashi formation, which averages perhaps 80 feet in thick- 

 ness. It is composed of the sands and sandy clays common in the Tertiary. It is distin- 

 guished by a characteristic bed of highly fossiliferous greensand with associated beds of lignite 

 immediately below it. By these features the Bashi may be easily identified across the width 

 of the State. The best exposure of the fossiliferous greensands of this formation is at Woods 

 Bluff, on Tombigbee River. 



