922 



EXPLORATION GEOPHYSICS 



These are characterized by the "mortar beds" which are often aquifers. 



The Smoky Hill River has its origin in eastern Colorado and flows 

 eastward. In this part of Kansas it has cut a wide valley through the 

 Tertiary cover down to the underlying Cretaceous Niobrara chalk forma- 

 tion. This allows the ground water in the Tertiary sediments to form 

 numerous springs along the river. Since the area is one of limited and 

 intermittent rainfall, these springs form the only firm and permanent water 

 source for the streams. 



The bedrock at the three sites was the Niobrara formation, which here 

 consisted of blue-gray massive chalk beds and alternating shaly chalk 

 horizons. The overburden at the sites varied from a sandy loam at higher 

 elevations along the proposed axes to sand and gravel in the river bottoms. 

 The stream itself was wide and slow-moving, indicative of old age and 

 aggradation of the channel. The river bed was from 150 to 200 feet wide 

 at the sites studied. 



Geophysical Results 



Elkader Dam Site. — The results of the bedrock depth determina- 

 tions by the seismic refraction method are shown in the profile in Figure 

 573, from which the surface elevations and bedrock depths may be deter- 



2700 



O 2600 



< 

 > 



2500 



DEPTH POINTS 



■ ^ o-o -^ ^V-*- BEDROCK 



1000 



2000 



3000 

 FEET 



4000 



5000 



6000 



Fig. 573. — Profile, Elkader Dam Axis. (Courtesy U. S. Depart- 

 ment of Interior, Bureau of Reclamation.) 



mined for points along the proposed axis. The depth points plotted along 

 the bedrock line were geophysically determined, and the rock line was 

 drawn between them in what appeared to be the most logical manner. 



The profile shows that the bedrock surface was covered by a relatively 

 thin overburden near the abutments which became thicker in the river 

 valley bottom. The maximum depth of 60 feet was reached at a point 3,100 

 feet from the left abutment. Across the valley the depth varied from this 

 maximum to 35 feet. 



Elkader Alternate No. 2 Dam Site. — The bedrock profile along the 

 proposed axis at this site is shown in Figure 574. It appears that the 

 overburden thickness is somewhat greater than at the Elkader site, as it 

 varies from 57 to 70 feet. As evident from a comparison of Figures 573 

 and 574, the conditions at the abutments at the two sites are very similar. 



