BULLETIN No. 37 86 



mate lono;itude, i 1-2 degrees east of Washington. Altitude at 

 Berwyn railroad station 495, to about 540 at hill crest, and 

 about 225 feet above tide at northeastern corner in the valley, 

 J am indebted to the United States v^/eather bureau for the mean 

 temperature, 68.68 degrees Fahrenheit and humidity 67, of 

 Philadelphia for the past three years from April to* August in- 

 clusive, which about covers the breeding season. It averages 

 a little cooler and dryer at Berwyn. 



South Valley hill, of which this block is but a small section 

 is the edge of a low table land (500 to 600 feet) is composed of 

 mica slate, doubtless of sedimentary origin and very ancient. 



This ridge slopes into the valley on the north, and on the 

 south forms but a part of the undulations to the Delaware val- 

 ley; as a whole however, ui^.broken for many miles and con- 

 stitutes the water shed between the Schuylkill and Delaware 

 rivers. Within the stated area, on the south side four brook- 

 lets have their source, unite into the Darby creek a mile or so 

 below, and flow into the Delaware below Philadelphia. On 

 the north, less than a quarter of a mile from springs of the 

 above, rise two branchlets of Trout run, which sparkling and 

 glancing in the sunlight obliquely across the green valley, min- 

 gles its waters with that of the Schuylkill below Port Kennedy. 

 At the extreme northern boundary a tiny stream has its begin- 

 ing, trickling down through many a shaded dell to join the East 

 Valley creek which in turn empties into the river at Valley 

 Forge. 



The northbound streamlets are invariably tlanked by the 

 steep ravine slopes, which in turn are cleft and seamed with 

 lesser ravines and gullies. Owing to an upheavel of the slaty 

 strata, the eastern side is always much more precipitous, often 

 with the rocks exposed, while the opposite side is covered with 

 earth. 



The Great Chester, or Downingtown Limestone valley, 

 as it is designated geologically, is a narrow basin averaging two 

 and a half miles wide, fifty-eight miles long and from two hun- 

 dred to four hunilred feet deep. The limestone overlies the 

 white or Potsdam sandstone, which represents the earliest 

 stratified rock in the country and lies near the surface on the 



