EFFECTS OF EKOSION. 43 
Ir would suspect that a difference of fully half a mile exists between 
I topography due to erosion and the original topography due to 
■net u re. 
ft very marked synclinal valley occurs on the western edge of the 
leen Mountain Range east of Lake Dunmore (PL XIII). This 
ricline begins at Sucker Brook and can bo, traced almost 5 miles to a 
pint 2h miles south of Silver Lake. It is in places U-shaped, and 
■•ms a wild glen, in which lies the carriage road from Brandon vil- 
te. The steep sides of this glen are from 300 to 500 feet high. The 
ftk is quartzite overlain by dolomite, which, however, has generally 
«n eroded from the sides of the syncline. Where the sides of the 
licline are very steep the quartzite has fallen in blocks, forming a 
jus covering the edges of the dolomite, 
RAVINES. 
} By far the most interesting pieces of land sculpture in the Taconic 
Ijrion are the ravines. To these is largely due the picturesqueness 
I the region as well as its abundant brooks and its dense vegetation. 
lese ravines are either parallel to the folds and then longitudinal, 
I transverse or diagonal to them, and frequently ramifying, so that 
le type passes into another. There are also amphitheaters resem- 
Jng glacial cirques or corries, except that they reach well down to 
fe valley floor. In the history of many of these ravines one or 
[other of the various systems of jointing (see p. 28) has helped to 
Itermine the course of a brook in its infancy, or, as erosion went on, 
le joint faces have fixed that course by guiding the stream. It is 
ttnmon to find the brooks flowing in these ravines bounded at inter- 
Ills by joints on one and sometimes on both sides. 
Although the ravines in the harder rocks of the Green Mountain 
linge and the Rensselaer Plateau do not ramify as much as those of 
le schist masses, yet some of them are so long and so deep as to imply 
[vast amount of time for their sculpture, A transverse cut from 
£0 to 1,000 feet deep, on the north side of Downer Glen in Man- 
tester, shows a quartzite fold, cleft by an east- west joint, which 
Irms the side of the ravine in that part of its course. The gorge of 
iiddlebury River is about 200 feet deep, but the transverse cut in 
e range of which this forms the narrower part is 2f miles broad 
ud extends from the 1,900-foot level down to the 1,000-foot one. 
lie lower part of the gorge is in places a canyon from 10 to 50 feet 
ide and 50 feet deep. The contact between the pre-Cambrian 
lieiss and the Cambrian schist crosses a pothole 40 by 50 feet wide 
the canyon. 
PL IX, A, shows the lower part of the end of a cut in the western 
Ige of the Rensselaer Plateau. The depth of the cut, measured from 
Bull. 272—05 m 4 
