CHAPTER XV 



THE BRYN MAWR SIPHON 



Geologic conditions as shown by exploration for a proposed pres- 

 sure tunnel 



Bryn Mawr is a railway station 2 miles northeast of Yonkers. 

 The general features of the vicinity, its topography, succession ot 

 formations and the boundaries are shown on the accompanying 

 sketch map which is largely copied from United States Geological 

 Survey Folio No. 83. The Southern aqueduct follows southward 

 along a Manhattan schist ridge until, at a point about a mile northeast 

 of Bryn Mawr, a cross depression of so great width and depth is 

 reached that some special means of crossing has to be devised. 

 Near Bryn Mawr station a gneiss ridge rises and continues south- 

 ward. The proposed line follows this ridge. 



Explorations have been made as usual by drilling to determine 

 if possible whether or not a bed rock pressure tunnel is practicable. 



The following questions may be made to cover most of the 

 practical issues of the study: 



1 What formations would the tunnel cut? 



2 Which of these would show most questionable ground? 



3 What portion of the line is regarded as most critical — whose 

 development would show whether or not a tunnel is practicable ? 



4 What special conditions are shown by drill borings ? 



5 What interpretation is to be placed on the peculiar results from 

 hole no. 4 where there has been unusually great difficulty in drilling? 



6 What experiences in similar ground have a direct bearing on 

 this case? 



Formations 



The formations that would be encountered in the Bryn Mawr 

 siphon are: 



1 Manhattan schist (top), the usual micaceous type, also called 

 Hudson schist in United States Geological Survey Folio 83. 



2 Inwood Hmestone (middle), the usual coarsely crystalline dolo- 

 mitic and micaceous type, also called " Stockbridge dolomite " in 

 the Folio, same as '' Tuckahoe marble," same as '' Sing Sing 

 marble," same as limestone at Kensico dam and also at Croton dam. 



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