Welcome to EvC Centrus,
The answer to this concern:
The Vishnu Schist, lying at the bottom of Inner Gorge, is said to be up to 2 billion years old. Now, the deepest point of the Grand Canyon is approximately 6,000ft or 1800m. Now, according to my limited understanding of geology, greater depth implies greater age, no? But, doing the math (and this is also assuming the Vishnu Schist is at the bottom of the Grand Canyon, as it may very well not be), this essentially means that every 1.8 micro metres is a year!
Now, a simple dust storm could cover centimetres of the top (at this point), no? And of course, a dust storm does not happen over 20 millenia! I know this is putting it extremely simply, but it was just an example I was given by a 15 year old fellow, and it makes sense.
The sequence of events in any given geologic column, such as the Grand Canyon, almost always consists of both deposition, where rock is added, and erosion, where rock already added is subtracted. Therefore depth is not a measure of time as rock layers were not always added.
Some places on Earth have been continuously eroding for quite some time, such as Australia and the Appalachian mountains, which have been since the Permian, some 250 million years ago.
At this site
http://www.kaibab.org/geology/gc_geol.htm is a summary of the depositional and erosional events at the Grand Canyon. As you can see, there were two times where the layers are not present in the column due to erosion that cover a significant period of time. One right above the Vishnu schist of 450 million years duration, and one of 250 million years duration between the Chuar Group and the Tapeats Sandstone. Also,
quote:
Rock layers younger than 250 million years have been eroded away and no longer exist in the immediate vicinity of the Grand Canyon.
Also, the layers of sandstone, limestone, shale, basalt, etc. that make up the Grand Canyon deposit at different rates and from different causes.
Hope that helps.
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