THE GENERAL PLAINS STEUCTUBE. 149 



Along the western portion of Section V, the Arapahoe, on account of 

 gentle dip and the removal of the overlying Denver beds, occupies a 

 broader superficial area than at any point along that outcrop to the north. 



The area covered lt\ the western portions of Sections 111. IV, and V 

 is discussed in the geology of the region about Golden. 



Till'. PLUM CRE1 K SECTION. 



The line of this section lies within a short distance of the northern 

 fair of the extensive table-land which forms the divide between the 

 Arkansas and Platte rivers. The western portion of the section presents 

 nothing unusual either in stratigraphy or structure; the formations succeed 

 one another with regularity, although the Montana has decreased somewhat 

 in thickness; the great fold at the foot of the mountains still appeal's in 

 the lower beds of the Arapahoe, the strata presenting the customary change 

 from steep to gentle dip and thence to horizontal well out beneath the 

 prairieS. A little south of the section, the broad zone of the Arapahoe, 

 Occasioned l>v the recession of the Denver formation north of Section V, 

 is again contracted by overlying Monument Creek beds, hut along this 

 portion of their outcrops the line of separation between the two formations 

 is of doubtful position, their materials being conspicuously alike. The 

 Denver in this portion of the field maintains its recession to the eastward, 

 and is in turn overlain by the Monument Creek. The broad area of 

 exposure of the Arapahoe beds possibly represents an old elevation in the 

 floor of the Denver sea. either isolated or a portion of the confining rim of 



the sea On the southwest. 



The relation between the Denver and Monument Creek formations is 

 distinguishable at several points along the line of section and in the bluffs 

 and high, rolling prairie to the south of it. In the bluffs the uneven surface 

 of erosion presented for the deposition of the younger formation appears 

 in a waving line, varying in the amplitude of its flexures from 50 to 150 

 or 200 feet, while in the southeastern part of the field and beyond there 

 frequently occur in the stream bottoms, and along their adjoining slopes, 

 local bosses of the Denver formation projecting through the overlying beds 

 of characteristic Monument Creek material. This is notably the case at 

 the head of Coal ('reek and on its tributary, Dutchman Creek. Between 



