FAULT SYSTEMS IN IOWA 111 



below the mouth of Lizard creek and half a mile below the 

 mouth of Soldier creek. Yet at the mouth of the Lizard the 

 limestone is abruptly encountered seventy Heel a'bove the water 

 level. It is an early observation of C. A. While and others that 

 no outcrops of gypsum occur for some little distance below the 

 mouth of Lizard creek; it is also a matter of early record thai 

 southward beyond the points just mentioned the gypsum sud- 

 denly appears in outcrop well up in the bluffs. For these 

 anomalies there has never been any adequate explanation of- 

 fered. As appears farther on, these features, together with 

 others, conclusively point to either abrupt flexing of the strata, 

 or notable dislocation in the continuity of the layers. Either 

 suggestion is a wholly unexpected phenomenon in this district. 

 In a region such as Iowa, where there is seemingly so little or- 

 ogenic disturbance, neither sharp folding nor extensive faulting 

 is ever appealed to. However, several extensive breaks in the 

 Iowa rocks are now known ; and other geotectonic features come 

 to light which give this phase of the State's geology a new trend. 



On the geologic map of Webster county,"' the nearly straight 

 line which the north margin of the gypsum-bearing field makes 

 is in itself suggestive of structural rather than erosion al causes. 

 This aspect of the areal limits was not thought of at the time 

 the map was drawn. The fact of its location shows how accu- 

 rately is the delineation notwithstanding the circumstance that 

 the reason thereof was unknown. 



As shown by outcrops and numerous well-sections located near 

 this line on either side, there is a marked discordance in the 

 meeting or matching of the various strata. 



As already indicated, the amount of displacement at the in- 

 tersection with the Des Moines river is not less than 100 feet. 

 This may or may not be the maximum throw ; probably it is not. 

 Several features point to a greater development of the fault 

 towards the southwest. 



The length of this great rent in the earth's crust is not yet 

 with accuracy determined. That it extends from Clarion, in 

 Wright county, to Wall Lake, in Sac county, a distance of eighty 

 miles, seems certain. That it is traceable beyond these points is 

 cpiite probable. It is safe to say that this fault is not less than 

 a hundred miles long. 



s Iowa Geol. Surv., Vol XII, 1902, p. l'J2. 



