Deep Notch, sometimes West Kill Notch, or Echo Notch, is a mountain pass in Lexington, New York, United States. It divides two Catskill peaks, both subpeaks of high peaks of the range. The narrow groove between the steep, high slopes on either side is traversed by state highway NY 42 and the Shandaken Tunnel, part of the New York City water supply system. It has been called "striking" and "a marvel of grandeur and beauty". The rocky slopes have required extra fencing at the base to prevent slides from blocking the road, the only paved road from northwestern Ulster County to neighboring towns in Greene County. Their steepness has also made them attractive to ice climbers, a recreational opportunity not widely available elsewhere in the Catskills.
"Deep Notch" is sometimes used to refer to the entire section of Route 42 from Bushnellsville at the county line to West Kill, where the road runs through the narrow valley of Bushnellsville Creek between 3,520-foot (1,070 m) Halcott Mountain on the west and 3,540-foot (1,080 m) Mount Sherrill, both High Peaks of the range, to the east. The actual notch is the roughly 1.5-mile (2.4 km) section north of the upper tributary of the creek to West Kill. On the west is an unnamed 3,408-foot (1,039 m) peak known as Northeast Halcott or Sleeping Lion; to the west is a 3,280-foot (1,000 m) false summit of Balsam Mountain, itself a subpeak of Sherrill.
The two generally level summits are separated by roughly one mile (1.6 km). Between them, the road crests at 1,900 feet (580 m) in the notch. On the west, that leaves a slope that rises 1,400 feet (430 m) to where it levels off below the summit of Northeast Halcott in 2,000 horizontal feet (610 m), complemented by a 1,300-foot (400 m) rise over a similar distance on the east. The grades are 70 percent and 65 percent respectively.
The steep pitch of the slopes has made Deep Notch attractive to ice climbers, who have mapped out several ice routes along the seeps of wintertime. They are generally rated as WI2–3 on the ice grading system, which puts them among the easier ice climbs. It is one of the few climbing areas in the Catskills, a range whose sedimentary bedrock makes its cliffs generally unsuitable for rock climbing, and one of the only ice climbing areas within a reasonable day's drive of New York City.