326 F. H. KNOWLTON CRETACEOUS-TERTIARY BOUNDARY 



adjacent Canadian territory. It is held that the dinosaur-bearing beds 

 above mentioned are separated from underlying beds by a major uncon- 

 formity which makes the logical line of separation between Cretaceous 

 and Tertiary. 



In eastern Montana, eastern Wyoming, and the Dakotas, where far 

 removed from the influence of the Eocky Mountain uplift, the forma- 

 tions involved are approximately horizontal. In a majority of cases 

 within this area the dinosaur-bearing Lance formation appears to rest 

 conformably on the underlying beds, and it is this condition apparently 

 wdiich has led many observers to deny the possibility of the existence at 

 these points of a time inten^al of any importance. It needs but a me- 

 mentos reflection, however, to show that because one formation lies in 

 apparent conformity on another, this is not of necessity proof positive 

 that the process of deposition continued uninterruptedly from the begin- 

 ning of the first to the close of the second. 



It may often happen that we must go outside the area where such ap- 

 parent confoiTQity obtains for the evidence which shall not only prove the 

 existence of the stratigraphic break, but also the value of the hiatus. It 

 happens, however, that even in the flat -lying beds in the Dakotas there 

 is some evidence of the measure of this time interval. The maximum 

 thickness of the Fox Hills — the formation beneath the Lance — is given 

 as 450 feet, yet in many places it is 75 feet or less, and in exceptional 

 cases appears to be entirely absent, and the Lance then rests oh Pierre. 

 An element of caution is necessary in interpreting this condition. Ine- 

 qualities in the thickness of a formation of the well known character of 

 the Fox Hills may be due to erosion or to irregularity of original deposi- 

 tion. That this unequal thickness in the Fox. Hills is actually due to 

 erosion and not to irregularity of deposition is indicated in at least two 

 ways: First, by the finding of actual erosion surfaces, as, for instance, 

 the one described by Calvert on Grand Eiver, South Dakota, where 

 within a horizontal distance of 500 feet there is an observed vertical cut 

 of at least 72 feet, arid other similar occurrences in western North and 

 South Dakota and eastern Montana, and, second, by the difference in the 

 invertebrate fauna in the lower and upper portions of the full Fox Hills 

 section, which is approximately 1,000 feet in thickness. In the type sec- 

 tion of Fox Hills at Fox Eidge, South Dakota, the beds show a thickness 

 of only about 325 feet. The fauna in this type section is said to show 

 more or less of a commingling of Pierre forms, and for this reason it was 

 at one time the inclination to abandon the use of Fox Hills as a distinct 

 formation and to regard it as merely a near-shore phase of the Pierre. 

 Where the full 1,000-foot section of Fox Hill& is present, as in the Denver 



