Despite all that an eagle's eyes can see, however, much more is known only to the humans. For unlike a bald eagle they can go back in their imaginations into geologic time, when warm seas, awesome mountain ranges, and icy glaciers arrived and departed from this land. Only they can learn about the Lenape Indians who hunted bear and moose here 10,000 years ago. Only they can investigate the grass, ferns, trees, and waters, and discover the birds, mammals, and fish that an eagle would miss.
What is the Delaware Water Gap?
What happened in this place long ago to create this landscape of rivers, hills, and valleys? Clues to the answer lie everywhere around us. By carefully exploring the land, geologists have put together a startling explanation.
Think big! Try to picture this: two separate Appalachian mountain ranges have appeared and disappeared in ages past. Snowy peaks of the first mountains reached their highest elevations during a geologic time called the Ordovician Period. That was between 500 and 425 million years ago (see the Geologic Time Line). These mountains were the first Appalachians, and like mountains everywhere they began eroding away into gravel and sand and mud. Rain, wind, and frost slowly wore the peaks down, leaving us today only their "roots," the deep interior of the old mountains. (You can see rocks of these old Ordovician mountains in the slate quarries near Slateford Farm and along the Interpretive Trail near Arrow Island Overlook. The slate was once sold for school slates, chalkboards, and roofing shingles.)
Near the mountains lay a large, warm sea basin, into which rivers washed the eroded gravel and sand and mud. Beneath the water this rock material, or sediment, slowly hardened into layers of sedimentary rock. Gravel turned into conglomerate; sand became sandstone; mud formed mudstone. And beneath the salty waves sea creatures lived and died just as they do in the sea today. Their lifeless shells fell to the seabottom to become fossils accumulating into layers of yet another sedimentary rock, called limestone.
These rock layers were deposited in the sea during the Silurian and Devonian Periods, between 425 and 350 million years ago. Except along Kittatiny Mountain and in the water gap, in all parts of the national recreation area you can find fossils in the rocks. A very common fossil is the brachiopod, a creature that had two shells but was neither clam nor mussel nor oyster. Snails lived here also. Reefs were built up from the limy skeletons of corals. Crinoids, primitive animals resembling upside-down starfish, lived on a stalk attached to the sea bottom. For a time giant eurypterids ("sea scorpions") patrolled the waters, keeping company with nautiloids, which were like octopuses in straight or coiled shells. Trilobites scurried over the sea bottom, and primitive armored fishes darted among the corals and crinoid stalks.
After the seas had collected vast layers of rocks and fossils, the second Appalachian mountain range began to form from them. Slowly the rock layers rose up out of the sea to become the folded Appalachian Mountains. By the Permian Period, about 230 million years ago, these new Appalachian Mountains were reaching their greatest heights, probably well over 3,000 meters high and comparable to today's Alps and Rocky Mountains. The layers of Silurian and Devonian rocks within them were being folded, bent, twisted, and crumpled. The seawater was pushed back, and on the fresh new land dinosaurs began foraging among the ferns, cycads (primitive, seed-bearing, non-flowering plants with large fernlike leaves), and conifers. All this required millions of years; folded mountains do not just explode from the earth.
Today we find no dinosaur fossils here. Why is this so? And what happened to those 3,000-meter peaks? You guessed it! Like the first Appalachian Mountain range, these new Appalachians have largely been worn away, including all traces of the dinosaurs that lived here. Today we see only ridges and valleys in the place of high mountains. The process of erosion has not stopped; every muddy stream that you see continues to carry away bits of the land.
You may wonder, when you visit the water gap, how the Delaware River managed to find or cut its way through Kittatiny Mountain, which is made up of very hard rock. The explanation is that the river was here before the mountain. Like the other mountains you see in this region, Kittatiny Mountain was formed by folding of the earth's crust. This folding occurred so slowly that the river was able to cut down through the rock layers faster than the folds were created across its path. The rock layers through which it cut are clearly visible in the gap, and there are several points at which the folding of the rocks is vividly displayed.
After the gap had been cut, there occurred an amazing event, the most recent major episode in earth's geologic historyContinental Glaciation. During the Ice Age, which spanned the last million years or so, the climate turned colder. Vast, thick ice sheets, or glaciers, accumulated from compacted winter snows. Under their own tremendous weight these sheets of ice spread slowly southward from the region of Hudson's Bay, killing everything that stood in their paths and grinding across the rocky hills beneath them. Advancing perhaps a few meters a day, the ice filled the valleys and overrode the heights. The scene was much like what you would see today on Greenland and the Antarctic subcontinent.
Then the climate slowly warmed again. The ice sheets began to melt. Trees and animals returned as the soil emerged into sunlight. Indians came and made this their homeland. About 11,000 years ago the ice had entirely melted away. Left behind were many signs of its visit: scratched and polished rock surfaces (you can see these commonly on Kittatiny Mountain trails); hills of stones and sand (called drumlins and kame terraces); and millions of pasture stones (called glacial erratics), which you can see where farmers have used them to build stone walls.
Today, the Delaware River is a broad path of quiet water. Islands such as Shawnee, Poxono, and Minisink dot the river. They are made of material that the river is too weak to wash down stream except during floods. And from the surrounding slopes, dozens of streams rush down to meet the river. These tributary streams are fast, turbulent, and full of spectacular waterfalls. This pattern of fast and slow streams in the national recreation area is typical of the Appalachian Mountains.
Last Updated: 30-Apr-2010