37G Frank Carney 



some lioi'izoDs arc inoi'c oi- less clayoy, othci's ui'(; quite calcareous. 

 The alternation of hard and soft horizons of rock is a factor in the 

 irrejj;ular to])of;Taj)}iy that characterizes the Allrghony plateau 

 throufi;!! i)art of Ohio. 



Relief and (/laciaiio7i. liut the irregular relief of this area is 

 the result of long contiiuied wc^atheiing. The agents of erosion 

 have act(Ml intennitteutly. Several drainage cycles have been 

 concerned ; one at least is thought to have worked itself to com- 

 pletion, or to base-level; the rest wei-e checked at different stages. 

 The result is the com])lex ])hysiogi'ai)hy, manifest particularly 

 in irregular drainage, which is due chiefly to stream divergence and 

 ca])ture, a subject discussed more fully in a later section fpp. 389— 



The su])erlicial dei)osits covering iii()r(> ihan. one-half of the 

 state are of glacial origin. This glacial drift is a mantle of weath- 

 ered rock in varying degi'ec s of comminution. Its oi'igin and the 

 (H)mplex ])rocesses involved in its t I'ansportation are more fully 

 considei-ed in the chapter dcNotcd to glaciation. No phas(^ of 

 the geologic history of Ohio is more im])ortaiit than glaciation, 

 which is the basis of the rich soil tlint has given the state its 

 rank in agricultui'e. 



(M'lOLOCIC TIMl': IM'IIMODS HIOPRESENTKD IN OHIO 



'{'he I'oeks of this state di]) gently to the south and (^ast. As a 

 result of this attitude, and of weathering since the sediments 

 wvYv deposited, the older formations outcroj) in the northern and 

 west (>rn ])arts of th(> state, and the youiigest in the southeastern 

 scM'tion. The rocks of the several jxriods, therefore, have a 

 sui'face arrangemeid, somewhat resembling that of shingles on a 

 roof, the ridge board being the axis of tlu" Cincinnati aix'h. 



Formations extending from the OrdoNician. to the Pleistocene 

 periods outcro]) in Ohio. Oidy by aid of the State Geological 

 Map caiitlu^ boundaries of these j)ei'i(i(ls be satisfactorily followed, 

 in the descri])tion below. This ma]) may ))e procured of the State 

 (i(>ologist, J. A. Bownocker, Columbus, for twenty-five cents; 

 (>very teacher in the stat(^ should have one at hand. 



Ordoviclan. The rocks of this j)eriod are on the surface in the 

 southwest corner of the state. Th(>y outci-oji along the Ohio 

 river east to t-lu^ valley of Brush creek, Adaius county, an.d north- 

 ward along the Gn^at JNliami river nearly to Piciua, from which 



