IN the northeast quarter of section 35, Portland Township, Cerro Gordo County, Iowa, lies Hackberry Grove, the type locality of the principal formation described in this paper. On the right (south) bank of Lime Creek i t this point is an escarpment which has an average height of about 75 feet above the normal water level of the creek. At the creek bed, extending upward for about 40 feet is a gentle, talus-covered slope, beneath which lies the clayey shale of the Sheffield formation. Above this shale, which is h e , plastic and of a uniform bluegray color, lie the blue, blue-brown and yellow shales of the Cerro Gordo division of the Hackberry stage. Above these are six to seven feet of limestones and shaley limestones which constitute the basal member of the upper, or Owen, substage of the Hackberry.In the case of this, as in many other formations, there is a lack of uniformity in names used; the name 'Lime Creek' commonly appears in place of 'Hackberry,' as well as in place of both 'Hackberry' and 'Sheffield.' Other names, such as 'Rockford Shales,' have been used, as well as the earlier 'Hamilton' and 'Chemung' of Hall and other geologists. The term 'Rockford Shales,' used by Webster in 1887, was preoccupied by 'Rockford Goniatite Beds,' and so had to be abandoned. In 1889 Webster proposed the name 'Hackberry,' to include all beds between the shale to which C. L. Fenton applied the name 'Sheffield' (1919) and the Kinderhook limestones and shales. In 1897 Calvin proposed the name 'Lime Creek,' retain-1 8 Hackberry Stage but the exposures available prevented him from securing accurate measurements. Calvin, in 1897, gave a section which, with the exception of the part dealing with the Sheffield Formation, was very accurate. The general section as taken by C. L. Fenton and C. L. Webster, and published by the f ~r m e r , ~ is given below : Feet 5. Acervularia Zone Calcareous, light-gray limestones, containing Pachyphyllum, Alveolites and other corals, with Acewdaria bassleri as the most distinctive species. Gastropods abundant. . . . . . .20
Floydia ZoneMagnesian shales and limestones and argillaceous dolomitic limestones, mostly deep buff or brown. Very fossiliferous, but with the fossils not very kell preserved. Numerous variants of the species of Floydia the characteristic fossils.. 30 3. Idiostroma Zone Bufi, buff-brown and gray limestones medium to thickly bedded, with some buff shale. Crowded throughout by two species of what appear to be stromatoporoids belonging to the indefinite genus Idiostroma. Floydia and brachiopods c o m m o n .
