36 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



boulder beds lying below the lake silts (Fig. 3). The proposed 

 tunnel would have passed out of the solid rock into the glacial till 

 and a change in plan was necessitated. Borings across the nar- 

 rows between Dempsey's Point and Marsh's Point showed a safe 

 thickness of rock above the tunnel but uncovered a gorge between 

 these two headlands (Profile Map — central part). Evidently the 

 pre-Pleistocene Bantam River flowed through a broad valley to 

 the north and a relatively restricted gorge under the present 

 narrows of the lake. The difference in the level of the bed rock 

 must be explained by depth of weathering or glacial scour or both. 

 South of the lake a seemingly trivial glacial divide blocked off the 

 original course of the stream and caused it to flow out of Bantam 

 Lake to the north within a short distance from where it enters the 

 lake. 



The original line of bore holes also shows that the long north- 

 south ridge between Bantam Lake and Cranberry swamp has no 

 corresponding rock ridge beneath it. The hill is composed entirely 

 of glacial debris and the old land surface rose only slightly from 

 the river's bank to fall again beneath the present swamp, then rose 

 steeply to the east to form high rocky ridges. 



Marsh's Point is also composed of glacial till though the rock 

 surface rises nearer to the present surface than in the first case. 



The extensive sand plain to the north of the northern expansion 

 of Bantam Lake lies at about the same level as the swamp and only 

 a few feet above the lake itself. The bore holes run south of this 

 plain so that the depth at which the bed rock lies is not known. 



Summary 



It may be stated in conclusion that this area was chosen for a 

 special report because of the construction of the tunnel that 

 exposed -niany features otherwise hidden from view. To be 

 sure, it is a critical area because the two schist series come 

 together, though, unluckily, an actual contact has not been found ; 

 but it is a difficult area to treat as an entity separate from the rest 

 of the western upland because it touches many problems that 

 cannot be solved in the area itself. The details are here presented 

 with full recognition that they represent only a part of a vastly 

 greater whole, but with the conviction also that they will aid in 

 the solution of the problems connected with them. 



The tunnel section and the examination of the outcrops in the 

 surrounding region have caused a number of changes on the 

 areal map. First, the Poughquag quartzite shown on the former 

 map around Bantam Lake is not present at all, but there are 

 many outcroppings of a quartz schist that forms a part of the 

 Hartland series. Second, the Becket gneiss formerly mapped 

 along the Shepaug River at the entrance of the tunnel is also 

 non-existent. That area is now tentatively placed as a part of 



