When the ammonites became extinct, the map of North America looked completely different: to the north, the Canadian Rocky Mountains already existed; to the south, the American Rockies had yet to rise. The date of the ammonites' extinction holds a key to when they first emerged.

These animals died about seventy million years ago in the middle of the Western Interior Seaway. And so we know at that time about seventy million years ago that this site was below sea level. So we know then that the Rocky Mountains had to rise from that seaway sometime after seventy million years ago.

Today all that is left from the ancient seafloor are these fossilized remains high in the Colorado Rockies.

Next, geologists needed to find out what pushed the seafloor up. The investigation moves to these slabs of rock flanking the Rockies just outside Denver, Colorado. They are known as the Flatirons, and they are part of the same formation that make up the Red Rocks' amphitheater. These slabs of rock are unusual because they contain holes, holes that make the Flatirons appealing to climbers and geologists alike.

So when we go climbing in the Flatirons, we are climbing on really nice handholds, in some cases, handholds that have been formed either by the pebbles in the rock or by zones of fine grained material that are easily removed by erosion the shales and the siltstones. Those layers get removed, leaving a notch for the hands to go in, and it makes for fantastic climbing.

The holes are a clue as to how these strangely tilted Flatirons were formed.

The layers themselves, the different grain sizes in the layers the silt, the sand, the pebbles this tells us that these are sedimentary rocks.

Sediments form in water when sand and small pieces of rock settle on the ground. Over millions of years they get compressed into layers of rock.

小编有约：呵呵~~看看我给大家准备的礼物。A. Denver, Colorado B. Rock C. holes
这里的线索能让你想起什么东西来？
来源:可可英语 http://www.kekenet.com/Article/201110/158188.shtml