ORDOVICIAN SYSTEM. In this area the upper formations of the Ordovician system are significant chiefly as horizon markers in deep-well sections. The readily recognizable character of one of them has greatly facilitated the interpretation of well logs and thereby the determination of structural features. The treatment here of formations not exposed within the area mapped has therefore primary reference to their position in the column and to the prominent characteristics which make them easily recognizable. ST. PETER SANDSTONE. The St. Peter sandstone (known also as the "Saccharoidal" sandstone, "First" sandstone, "Crystal City" sandstone, "Pacific" sandstone, and "Cap au Gres" sandstone) is composed almost entirely of well-rounded grains of generally translucent quartz, medium to large in size and poorly cemented with silica. Partly because of the deficiency in cement and partly because of the roundness of the grains, the rock is very friable. It may be almost pure white or stained with iron oxide to various shades of yellow, pink, or even red. In places the iron stain occurs in irregular blotches or incipient concretions. The thickness of the formation at Pacific, 34 miles west of St. Louis, is at least 110 feet. (See Pl. III, A.) The well at the insane asylum, St. Louis, is believed to pass through 133 feet of this formation. The thickness, however, varies from 200 feet down to the vanishing point, the sandstone being unconformable both above and below. FORMATIONS ABOVE THE ST. PETER SANDSTONE. Overlying the St. Peter sandstone, and like it not outcropping within the area mapped on Plate I, are 600 to 800 feet or more of limestones and shales which were not studied in detail. They represent the Plattin, Joachim, and Kimmswick (Ordovician) limestones, the Maquoketa shale (upper Ordovician), and possibly also some Devonian shale and limestone. The limestones may be seen in descending order in outcrops or railroad cuts in going from Fern Glen, 20 miles west of St. Louis, to Pacific Junction, 14 miles farther west. Both the limestones and the overlying shales are shown in railroad cuts south of Kimmswick, which is 20 miles south of St. Louis. These formations occupy in the Belcher well section the interval from 725 to 1,505 feet, in the asylum well from 883 to 1,452 feet, in the Monks Mound well from 1,095 to 1,975 feet. A. ST. PETER SANDSTONE CAPPED BY LIMESTONE, PACIFIC, MO. B. CROSS-BEDDING IN SPERGEN LIMESTONE, MERAMEC HIGHLANDS, MO. (See pages 22-23.) CARBONIFEROUS SYSTEM. MISSISSIPPIAN SERIES. KINDERHOOK FORMATION. Like the Ordovician formations named above, the Kinderhook does not outcrop within the area mapped. Its prominence in the well logs, however, requires its description in some detail. It contains several distinct members, arranged from above downward as follows: 4. Greenish-gray cherty limestone (supposed base of Burlington, but 3. Pink calcareous cherty shale and limestone, with locally a 4-foot mas- 2. Yellow friable sandstone (Bushberg sandstone member, local). 1. Gray limestone, in many places containing sand grains (Glen Park limestone member, local). These members of the Kinderhook are well exposed at Glen Park, 23 miles south of St. Louis, where the section is as follows: 11. Cherty limestone (Burlington). Section at Glen Park, Mo.a Ft. in. 18 10. Cherty greenish-gray limestone (Burlington) [may contain some 7 9. Red, more or less argillaceous, limestone and chert with green- 8 6 8. Unexposed, probably a softer red calcareous shale covered 6 7. Hard, somewhat crystalline yellow or gray limestone [undoubt- 4 6. Yellow fine-grained sandstone, becoming brown on the weath- 14 5. Sandy shale..... 4 4. Limestone, coarse, light gray, oolitic, and fossiliferous (Glen 3. Dull yellow shales and shaly limestone (Maquoketa).......... 1. Kimmswick limestone (Trenton fossils). 1 6 55 The upper Kinderhook is likewise typically exposed at Fern Glen, 20 miles west of St. Louis, and from this place the name Fern Glen limestone member is taken. The section at Fern Glen is as follows: Section at Fern Glen, Mo.b 5. Cherty limestone (Burlington; may contain some Kinderhook Feet. (?) 4. Greenish calcareous shale, with chert bands and nodules, becom- 14 3. Red calcareous, highly fossiliferous shales, with chert and some 12 2. Pink to red cherty limestone... 1. Buff to greenish limestone with crystalline crinoid stems.. 14 a Weller, Stuart, Kinderhook faunal studies: Trans. St. Louis Acad. Sci., vol. 16, 1906, pp. 435-438. The notes in brackets are supplied by the present writer. b The measurements in this section are taken from Weller's unpublished notes. Beds 1 to 4 in the above section constitute the Fern Glen limestone member of the Kinderhook formation. The cherty limestone above the shale, between the Osage group and the Fern Glen limestone member, is physically continuous with the overlying Osage, from which it is with difficulty delimited because of the lack of a sharp break in the character of the fossils. The thickness of this limestone, therefore, which should be classed with the Kinderhook, is uncertain. No. 3, in the general section of the Kinderhook (p. 19), is the Fern Glen limestone member and is found wherever this horizon is exposed or drilled through, in and near the area studied. Its striking characteristics are its pink or red color and its large proportion of shale. At Fern Glen (see p. 19) it grades from a pure massive limestone at the base to shale at the top. Its red color is most pronounced in the middle, and it is cherty throughout, with the exception of the massive limestone at its base. The appearance of the Fern Glen limestone member at Glen Park (Nos. 7 to 9 of the section at that place) is essentially the same, though limestone forms a somewhat greater proportion of the whole than at Fern Glen. Among the characteristic fossils of this member are Zaphrentis (several species), Cyathaxonia sp., Evactinopora sp., Leptana rhomboidalis, Rhipidomella (small species), Spirifer vernonensis, Spirifer imbrex, Athyris prouti, and Athyris lamellosa. OSAGE GROUP (BURLINGTON AND KEOKUK LIMESTONES). Overlying the Fern Glen limestone member is from 200 to 250 feet of cherty limestone doubtless comprising both the Burlington and the Keokuk limestones. Because of the continuity of character throughout and the absence of a decided paleontologic break, all these beds are treated together here under the more comprehensive term Osage group. The outcrops of this group within the area mapped are limited to the southwest corner of the quadrangle, in the valley of Meramec River. In this area only the upper 100 feet is exposed and all the fossils found are of Keokuk age. The limestone is characteristically light colored and thin bedded, the thickness of the several beds being generally a fraction of a foot, but in extreme cases reaching several feet. The one conspicuous physical characteristic of these rocks is the great abundance of chert. Throughout their greater part chert constitutes approximately half of the mass. The chert is distributed in beds, irregular zones, and bands of concretions more or less separated into individual units, some elliptical and even spherical. The prevailing form is in beds of irregular thickness and wavy surfaces, so that on a casual view an exposure gives the impression of irregularly interbedded limestone |