The Sly Gap Formation is a geologic formation in south-central New Mexico.

It preserves fossils dating back to the Frasnian Age of the late Devonian period.

[1] The formation consists of about 114 feet (35 m) of brown, yellow, or gray limestone and siltstone and brown to greenish-gray shale.

[2] It pinches out in the west in the Basin and Range province.

[1] The formation includes crinoidal siltstone beds and many other beds are fossiliferous, bearing crinoid, brachiopod, bryozoan, anthozoan, cephalopod, gastropod, fish, and coral remains.