The Goosenecks of the San Juan River in southeastern Utah are considered to be the best examples of entrenched meanders in the world. Meandering streams normally occur on nearly flat, low-slope land surfaces and have a highly sinuous path. Southern Utah's topography is quite rugged - steep slopes are common. The presence of a meandering stream in such a rugged landscape seems paradoxical. The current landscape in southern Utah formed by tectonic uplift of the Colorado Plateau Physiographic Province during the Cenozoic Era. Pre-existing streams on relatively flat landscapes retained their channel courses as they eroded downward through the uplifting plateau.

The bedrock walls along the San Juan River consist of the Honaker Trail Formation (= upper two-thirds of the wall) over the Paradox Formation (= lower third of the wall, down to river level) - both are Pennsylvanian in age. The Honaker Trail is mostly shale and limestone, plus some sandstone. The Paradox is mostly limestone.

From park signage:

Approximately 1,000 feet beneath the spot where you now stand winds the San Juan River. Originating in Colorado, this river eventually joins Lake Powell.

The silt-laden San Juan has been instrumental in cutting the deep bending chasm directly below. This section is called the Great Goosenecks of the San Juan River.

Geologists consider this part of the river to be one of the finest examples of “entrenched meanders” anywhere in the world.

The meandering pattern originated several million years ago when the river was flowing on a relatively flat plain, much as the present-day Mississippi River.

The San Juan became entrenched when the entire Colroado Plateau was slowly uplifted. Cutting downward, the river followed its initial pattern and thus created the canyon you now view.

The process continues to this day as the San Juan River cuts ever deeper into prehistoric geological formations.