iJ2 The American GeoUxjist. August, ij*96 
To completely demonstrate the a^v of peiM-plaiu No. 1, it 
will be necessary to trace it westward to the edge of the Cre- 
taceous strata in northern Iowa. But it has been mainly 
destroyed west of the plateau bounded by the "Niagara 
escarpment" in northeastern Iowa, and the country is so 
heavily drift-covered that the preglacial topography is some- 
what obscure. Close to the village of Hockville, in Delaware 
county, Mc(iee describes* a conglomerate formation resting 
on the Niagara limestone and overlain by drift, and he corre- 
lates it. largely on lithologic evidence, with certain Cretaceous 
outliers in southeastern Minnesota. After considering the 
evidence, he says: "On the whole it is safe to provisionally 
refer the Rockville conglomerate to the ('retaceous, and to 
more doubtfully correlate it with the Nishnabotany sandstone 
of southwestern Iowa, and (at least approximately) with the 
Fort Dodge gypsum of western central Iowa." Unfortunately, 
it has as yet yielded no fossils, so that its Cretaceous age is 
not positively proven. However, if it is a marine formation, 
its high position precludes the probability of its being a Car- 
boniferous or Devonian outlier; and the absence, so far as 
known, of a Tertiary sea in Iowa would seem to support its 
Cretaceous age. In regard to its position, McGee further 
says: "It crowns one of the salients separated by the nar- 
row gorge of the North Maquoketa from the Niagara upland 
which extends thence nearly to the Mississippi river." Its 
altitude is too great for peneplain No. 2, but it appears to 
bear such a relation to peneplain No. 1 that they may be cor- 
related with each other in a general way. 
In southeastern Minnesota undoubted n:arine (h-etaceous 
deposits occur at levels varying between the 1,000 and 1,150 
foot levels. This is in a district not very distant from the 
area occupied by the "mounds," and unless there has been a 
very great uplift of tiie Cretaceous baselevel in the district 
adjoining, on the southeast, that in which these Cretaceous 
outliers are found, we may expect to find this peneplain over 
the Driftless Area, — if any remnants of it exist, — at about 
the level of the summit plane of the " mounds." In short, all 
the evidence, although fragmentary, seems to lead us to the 
following conclusions: {a) if there is a baselevel plain of ero- 
*Eleventh Annual Report, U. S. Gaological Survey, Part i, p. 304. 
