434 BOTANICAL GAZETTE [November 



is the Holyoke diabase. These Triassic rocks were later subjected 

 to tilting, warping, and faulting, and these processes, together with 

 stream erosion, resulted in the formation of the "trap" ridges 

 which stand above the valley floor with steep cliffs facing the west 

 or the north, as the case may be, and more gentle slopes on the east 

 and the south, slopes that are practically that of the dip of the 

 rocks. These cliffs are in places 600 feet high and in others are 

 almost buried by the accumulation of talus and glacial debris. 

 There are numerous faults running north and south, in general 

 crossing the Holyoke Range at right angles and running almost 

 parallel with the Mount Tom Range. 



During the ice age the ice sheet passed over the region and left 

 much glacial material on the north side of the range. As the ice 

 retreated north a lake was formed here and another one on the 

 south side. The one north of the range was called the Hadley 

 Lake and the one south, the Springfield Lake. 



The Hadley Lake found an outlet between Nonotuck and Titan's 

 Pier (see map). Today the area is drained by the Connecticut 

 River, which doubtless has occupied different portions of the lake 

 bed, but now is cutting close to the east side of the Mount Tom 

 Range (see map) . 



The region is being acted upon but little by stream erosion, as 

 the streams are of small size. On the south side of the Mount 

 Holyoke Range there formerly were 20 streams originating at an 

 elevation of 400-500 feet, while on the north side there were 6 at 

 that height. The many streams on the south side of the range 

 were doing active erosion work along the fault lines, but the fre- 

 quent deforestation has been instrumental in exhausting the 

 streams, and as a result there are many ravines, young topographi- 

 cally but quite old floristically. 



On the north side of the Mount Holyoke Range there are wide 

 crevices in the cliff face due to the presence of faults, but below 

 the cliff face the mountain side presents a uniform mass of talus 

 and glacial debris. Along the talus below the wide crevices there 

 is always a greater amount of moisture, where more of the "run 

 off" on the north side of the range sinks into the talus than along 

 the talus at the foot of the unbroken cliff face. 



