256 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



called Plateau region of central Alaska, being the intermontane belt 

 bounded on the south by the ranges of the Pacific mountain system and 

 on the north and east by the Eocky Mountain system. 



The topography is characterized by broad, flat-topped, interstream 

 areas, separated by the valleys of a well developed drainage system. The 

 valleys are usually broad and flat, with gentle slopes. Certain exceptions 

 to this characteristic valley type will be noticed below, but of especial 

 importance are the ramparts of the Yukon, lying below the great Yukon 

 flat, and the ramparts of the Porcupine. These topographic forms are 

 probably in part due to recent^ local warpings, in part to the influence on 

 erosion by character of bedrock. 



The dominating feature of the topography is the baselevel character of 

 the upland summits, which represent an uplifted and dissected peneplain. 

 This old land surface is deeply mantled by the products of weathering, 

 while the valley slopes are often buried under a heavy talus. Therefore 

 outcrops of bedrock are not common. The difficulties of deciphering the 

 rather intricate geology of this field lies not so much in its inaccessibility, 

 as is generally supposed, as in the absence of exposures of bedrock. In an 

 overland journey the geologist will often travel for days without seeing a 

 single outcrop, and must perforce map the boundaries of the stratigraphic 

 units solely on the facts gleaned from the weathered surface materials. 

 The general absence of excavations and the heavy coating of moss which 

 nearly everywhere mantles the soil increase the difiiculties of geologic 

 mapping. Fortunately the exposures of bedrock are not uncommon along 

 the large watercourses, and this fact alone would justify the publication 

 of this paper, which is based largely on the observations made along the 

 valleys of the Yukon and the Porcupine rivers, supplemented by the 

 results of overland journeys in part made in previous years. 



Geologic Investigations 



During the past three decades many Avorkers have taken part in the 

 investigation of this general field, and the conceptions of the stratigraphic 

 succession have gradually crystallized into definite form. The character 

 of these studies has varied from the exploratory journeys of Kennicotr, 

 Dall, Ogilvie, and McConnell to such detailed examinations of special 

 areas as have been made by Collier and McConnell. These various in- 

 vestigations will be referred to below, and to all of them the writers are 

 indebted for clues to the geology of the region. 



2 Alfred H. Brooks : The Geography and Geology of Alaska. Professional paper no. 45, 

 U. S. Geological Survey, 1906, pp. 278-282. 



