The Gap

The Delaware Water Gap is bounded by Mt. Minsi in Pennsylvania and Mt. Tammany in New Jersey. Once touted as a scenic Wonder of the World, it is an impressive site when motoring through on Interstate 80 or viewed from the overlooks along Rt. 611 on the Pennsylvania side.

The Ridge

Before the Gap formed. Mt. Minsi and Mt. Tammany formed one continuous ridge -- the Kittatinny Ridge, along which the Appalachian Trail runs through the park today. Here at the Gap, this ridge is composed of two different sedimentary rock formations: the Shawangunk Formation, and the Bloomsburg Redbeds.

The Rocks

The Shawangunk Formation is the older of the two and forms the bulk of the mountain. It is composed of three layers -- or members. The top and bottom members are largely sandstone, with little shale. The middle layer has a mix of weaker shales and sandstones, and this is where trees grow.

The Bloomsburg Redbeds are more recent and cover the top of the mountains. (See Geology Stops on Mt. Tammany at right) They consist of layer upon layer of sand and mud that later became stones and shales of the formation.

The Mountains

Although they now tilt upwards at a precarious angle, the Shawangunk and Bloomsburg Formations were laid down horizontally. When the African Plate collided with the North American Plate 250 million years ago, the force of the collision bent the rock layers into folds, forming the Appalachian Mountain chain. Erosion has removed the rock from what is now the Water Gap.

The Work of Water

There are several explanations of how the Water Gap formed, but essentially, erosion has removed the rock from what is now the Water Gap

Headwater Erosion (and stream-capturing)explains that the folding of the rocks layers resulted in a weakened spot in the ridge. Creeks flowing down the mountain eventually slowly eroded their way back through the ridge until they cut the whole way through and "captured" a river on the other side of the ridge. (See How the Gap Formed at right)

Superposition explains that creeks eroded an ever deepening channel; the folding or weakening of the rock is not involved.

Ice Comes and Goes

Glaciers have come to this valley and melted away several times over millions of years, the last (Wisconsin) glaciation having melted away 20,000 years ago.

While glaciers likely carried away rock from the slopes, the Water Gap was already by the time that the glaciers advanced. There are glacial erratics (boulders) and till deposits in the valleys and on the mountains, and glacial striae (scratches) on the walls of the Gap itself. (See Geology Stops on Mt. Tammany at right)

A Work in Progress

Though in the human frame of time, the Gap looks "finished," erosion is ongoing. Even if you don't know one rock from another, or one geologic era from another, take a moment to ponder the power of water over immense periods of time -- water is still forming the Gap, drop by drop, as you watch the river flow by today.

What's a Water Gap? (Fun Page for Kids - pdf file)