104 IOWA ACADEMY OF SCIENCE 



southeast. The amount of displacement is large. The spacing 

 is wide. The ruptures are long and somewhat curved. In the 

 other system, which is confined to the western portion of the 

 state, the value of the movement figures is not nearly so great 

 as in the case of the other; yet it is still quite notable. The 

 space beween successive faults represents a distance of about 

 twenty-five miles. Both systems of faults appear to have greater 

 displacement values outside of the state and to vanish within 

 the boundaries of our commonwealth. The faults extend lat- 

 erally far into neighboring states. (Plate IV.) 



At this time it is not necessary to go exhaustively into des- 

 criptive details concerning the individual faults, since these 

 features are in another connection subject of extended discus- 

 sion. The most conspicuous of the displacements is the Cap- 

 au-Gres fault. It is the most notable line of recent dislocation 

 found anywhere in the Mississippi valley. Its salient features 

 are best displayed on the Mississippi river near the mouth of 

 the Illinois river. The sandstone headland which marks its 

 position there has been a. prominent landmark to early voya- 

 geurs and rivermen for a period of more than two and a half 

 centuries. The upturned edges of the strata constitute one of 

 the most extensive and complete geological sections on the con- 

 tinent. "Within a distance of one short mile along the river 

 bluff, at Folley station, the entire Paleozoic succession, from 

 Cambric dolomites to Coal Measures, is exposed. Measurements 

 indicate a vertical displacement of more than 1,000 feet. So 

 admirably are the disturbed rocks displayed that a photographic 

 print of the bluff clearly retains all the structural features. 3 



The line of the Cap-au-Gres rupture extends from Leon, in 

 southern Iowa, to Vincennes, Indiana, a distance of 400 miles. 

 At its eastern extremity the fault passes into a fold, probably of 

 monoclinal rather than anticlinal character, that gives rise to 

 the great oil reservoir of eastern Illinois and western Indiana. 

 The western extension likewise passes into a fold which furnishes 

 the most favorable conditions in our entire state for tVie occur- 

 rence of oil and gas. At the southern boundary of Iowa the 

 line of this fault is conspicuously marked on the surface of the 

 ground by a long eastward protrusion of the Bethany lime- 

 stone, the basal terrane of the Missourian series. This tongue 

 carries the Bethany formation a distance of fifty miles beyond 



3 See Proc. Iowa Acad. Sci., Vol. V, 1898, p. 5S. 



