60 TOPOGRAPHIC DEVELOPMENT OF KLAMATH MOUNTAINS, [bull. 196. 
Tli is tidal transgression has been regarded as indicating that the 
coast has subsided. That such is the case is shown also by the fact that 
the rocky beds of the stream are now far below tide level. Borings 
have been made on the borders of Isthmus and Kentuck sloughs, and 
the slough muds were found to extend down at least 200 feet below 
tide — possibly much farther — indicating that at some former epoch 
that part of the country stood at a higher level than at present, 
enabling the streams to cut out the portion of the valley which is now 
submerged. The extent of this subsidence is not definitely known. 
RELATION OF KLAMATH PENEPLAIN TO THAT OF SIERRA 
NEVADA AND COAST RANGE. 
On the eastern side of the Sacramento Valley lies the Sierra Nevada, 
whose long, gentle slope to the valley presents a remarkable plain of 
erosion. Much has been written concerning this feature, but one of 
the latest and most noteworthy contributions is b} 7 Mr. Lindgren, 1 
who recognizes in it two plains of erosion, the one which is most com- 
plete consummated just before the deposition of the Chico 2 (Creta- 
ceous), the other during the Miocene. Mr. Lindgren says: 3 
The relation of the two eroded surfaces, the Cretaceous and the Miocene, is 
clearly discernible from any point in the lower foothills looking up toward the 
summit of the range. Above the deep canyons of the modern gorges extend the 
broad, flat lava plateaus, capping the separating ridges and looking very much 
like an old base-level. These lava flows cover the comparatively gentle topography 
of the Miocene valleys. Above them rise the peaks and ridges just mentioned, 
and indicate with their level sky line the 'extent of a far older eroded surface 
uplifted and dissected long before the auriferous gravels were deposited or the 
lava flows extruded. 
The Cretaceous age of the earlier plain pointed out by Lindgren 
finds support in the character of the Cretaceous sediments of the Sac- 
ramento Valley. On the western side of that valley the Lower Cre- 
taceous beds are mostly fine shales with calcareous nodules and are of 
great thickness, indicating a wide range of low land as their source. 
During the Chico, which immediately followed, there was an epoch 
of marine transgression and possibly greater land declivity, for near 
the base of that series is a heavy conglomerate which can be traced 
for many miles about the borders of the Sacramento Valley. The 
final sediments of the Chico were such as to indicate low relief. 
The condition of the Klamath Mountain region during the progress 
of the Cretaceous is not yet fully understood, owing to the fragmen- 
tary nature both of the Cretaceous deposits and of our knowledge 
of them. However, the known distribution of the Cretaceous rocks 
among the Klamath Mountains, taken together with the border of 
Cretaceous sediments which may be traced almost continuously 
around them, indicates that during a late part of the Cretaceous 
1 Jour. Geol., Vol. IV, 1896. p. 881. 2 Idem, p. 894. 3 Idem, p. 897. 
