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SCIENCE 



[N. S. Vol. XXVII. No. 695 



has familiarized geologists with the great 

 horizontal wedge of Proterozoic (Algonkian) 

 sediments which thins out to the west just 

 south of Vishnu Temple, but there is nothing 

 in the literature of the Grand Canyon which 

 prepares the visitor for encountering a mass 

 of the same sediments some ten miles west of 

 Vishnu Temple and directly opposite the hotels 

 at the end of the Grand Canyon Railroad. 



Fig. 1. Sketch from Bright Angel trail look- 

 ing northward across the inner gorge of the Colo- 

 rado and up Bright Angel Creek. The Bright 

 Angel fault is shown on the right crossing a spur 

 of gneiss capped by a small mass of Proterozoic 

 sediments. 



Doubtless other eyes than mine, practised in 

 geological observation, have recognized the 

 Proterozoic rocks in that part of the northern 

 wall of the canyon lying just west of Bright 

 Angel Creek, for the beds are visible from 

 El Tovar hotel. It seems appropriate, how- 

 ever, that there should be published some ac- 

 cessible note calling attention to the interest- 

 ing features there displayed, for the benefit of 

 the thousands of people not necessarily geol- 

 ogists, who study with intelligent interest this, 

 the most frequented part of the great chasm. 

 Although the general relations of the Pro- 

 terozoic rocks can be made out from the 



southern rim of the canyon they may best be 

 studied from the edge of the Tonto platform 

 just west of Pipe Creek and overlooking the 

 inner gorge — a vantage point easily gained by 

 leaving the traveled Bright Angel trail at 

 Indian Garden and walking or riding north- 

 ward over the comparatively smooth upper 

 surface of the Tonto sandstone (see Fig. 1). 

 From this place the structure shown in the 

 accompanying section (Fig. 2) is superbly dis- 

 played. The long straight gorge of Bright 

 Angel Creek coincides with a fault of which 

 the throw is at least 300 feet, the west side 

 being relatively depressed. This fault-zone is 

 visible at many points on the south side of the 

 canyon, being crossed and recrossed by the 

 Bright Angel trail as it zigzags down the steep 

 slopes of gneiss along Pipe Creek. On the 

 east side of the gorge the much contorted, 

 truncated, Archean gneiss is capped by hori- 

 zontal " Tonto sandstone " conformably over- 

 lain by the "Tonto shale" and the "Red 

 Wall limestone " ; on the west side the red 

 san(fatones and shales of the " Unkar terrane " 

 rest, also uncomformably, upon the Archean 

 with a low dip to the east. About two miles 

 west of the mouth of Bright Angel Creek the 

 pre-Cambrian sediments are cut off by a sec- 

 ond fault which, as shown in the section and 

 sketch, does not displace the overlying " Tonto 

 sandstone." 



A partial and brief summary of the history 

 of events recorded in this section is as follows : 

 (1) The reduction of the Archean rocks to a 

 plain of erosion. (2) The deposition of the 

 Proterozoic sediments, (3) Faulting, by which 

 a mass of the sedimentary rocks was inset 

 into the Archean. (4) Peneplaination of the 

 region. At the end of this erosion period, part 

 of the inset block of pre-Cambrian rocks was 

 left as a low monadnock above the general 



Fig. 2. Diagrammatic section of part of the north wall of the Grand Canyon near the mouth 

 of Bright Angel Creek. Vertical and horizontal scales the same. 



