No. 40] GEOLOGY OF SHEPAUG TUNNEL 35 



the tunnel. The remaining granite dikes that occur in the tunnel — • 

 to some extent in the diorite but to a much greater degree in the 

 Hartland schist — are granitic textured rocks that have the same 

 composition as the gneiss noted above but lack its foliation. Great 

 quantities of pegmatite dikes, large and small, cut the Brookfield 

 diorite and the Berkshire and Hartland schists throughout the 

 tunnel. Some of these dikes show a distinct foliation and small 

 ones are apt to be folded with the schists, but the majority are 

 massive and cut across the foliation planes of the older rocks. 



Buried Surface Features Exposed by the Tunnel and the 

 Diamond Drill Holes 



It is not the purpose of this report to describe the glacial 

 phenomena of the region in detail but certain features that have 

 been exposed to view by the tunnel excavation or by the diamond 

 drill holes that preceded the excavation can properly be included. 



pre-pleistocene weathering preserved south of mount 



prospect 



It is generally true in this region, as would be expected in a 

 recently glaciated area, that the rock underlying the rather thin 

 cover of till has undergone very little weathering since the products 

 of pre-Pleistocene decay were swept away by the advancing ice. 

 There is one notable exception to this rule immediately west of 

 the peat bog that lies in the path of the tunnel fifteen hundred 

 and sixty feet west of the Bantam River crossing. For a distance 

 of nearly two thousand feet the tunnel was driven through soft, 

 rotten rock that made considerable trouble for the engineers since 

 it necessitated continual timbering. A number of small faults 

 occur in this part and both the diorite and biotite-hornblendite are 

 crushed and slickenslided in several places, but these faults are 

 neither large enough nor common enough to be the direct cause 

 of the weakness of the rock, though they may have helped to 

 cause the unusual depth of decay. This area of deep pre-glacial 

 weathering was apparently partly protected from glacial erosion 

 by the higher lands to the north and some part of the weathered 

 rock was preserved. 



THE PRE-GLACIAL SURFACE UNDER THE NORTH END OF BANTAM 



LAKE 



The diamond drill holes that were bored into the bed of the 

 lake to determine the thickness of the rock cover above the pro- 

 posed tunnel line directly across the northern part of the lake 

 showed a surprising depth of sand, gravel and heterogeneous 



