424 
STRATIGRAPHICAL GEOLOGY 
The evidences thus seem conclusive that the gravel bed under 
consideration forms the basal member of the Chadron formation, 
and is Oligocene in age. It represents initial deposition of the 
regional Tertic succession. 
Limitation of Cretacic Formations in Southwestern Iowa. Defin¬ 
ite location of important fault lines ^ in Iowa brings about exten¬ 
sive rectifications of many of the previously accepted geological 
boundaries. Areal distribution of the Dakotan sandstone in the 
southwestern portion of the State is particularly aflfected. A very 
considerable tract formerly mapped as Cretacic in age must now 
be changed to Carbonic colors. Extensive tongues of Cretacic 
deposits once regarded as parts of the main body are now 
known to be merely outlying patches far removed from the con¬ 
tinuous mass. 
When Lonsdale ^ was engaged in the mapping of the county of 
Montgomery, a representative tract in southwestern Iowa as it 
was thought, and an area that would later serve as a standard for 
the surrounding districts he was completely nonplussed because of 
Fig. 25. Red Oak Fault in Southwest Iowa 
the fact that in the northern part of the county he was unable 
to locate a single outcrop of Cretacic rocks. The strata of that 
age of the southern portion of the district abruptly ceased to show 
themselves after passing a certain line, a short distance north of 
the town of Red Oak. Leaving the State after writing the report 
and in a slightly unfinished condition others filled in the narrow 
uncompleted belt with Cretacic colors. 
1 Proc. Iowa Acad. Sci., Vol. XXIII, p. 103, 1916. 
2 Iowa Geol. Surv., Vol. IV, p. 381, 1895. 
H. J. Cook. 
