1) "Unstratified rock such as granite is lifted to the earth's sruface. Once exposed, the granite begins eroding. Sediments are then deposited on the eroded surface. The boundary between the granite and the sedimentary rock layer above is a nonconformity. It represents an unknown period of time during whcih the granite was eroded."

I don't get why this (called nonconformity) is considered to be a type of unconformity, I mean, why is the period of time during which the granite was eroded is unknown? It is simply eroded before the sedimentary rock layer above is deposited, true?


2) "The edge of the Canadian Niagara Falls is eroding at an average rate of 1.3 m per year. Based on the average rate of erosion, scientsts then determined that the falls probably formed about 9,900 years ago."

If the edge is eroding at an average rate of 1.3 m/year, wouldn't the fall be 1.3 x 9900 = 12870 m taller than it is now? If so, that tells nothing about when it formed, how do they use the average erosion rate to determine that the fall formed 9900 years ago, then?

Would anyone like to discuss? ;)

1) Let me answer your first question with a question: how would you know how long the granite had been eroding for? One year, one millenia, one million years, ten million years?

2) The Falls have carved a gorge. It is the length of the gorge that allows us to compute the approximate age.

2) In my text book, it says that the absolute age of a sample can be estimated using its average erosion rate, and then it gives the Canadian Niagara Falls as an example, saying that the edge (the top of the fall, I guess?) is eroding at an average rate of 1.3 m per year. Therefore, scientsts are able to estimate that the falls formed about 9,900 years ago.

(i)The edge of the falls is eroding at an average rate of 1.3 m per year
(ii) able to estimate that the falls formed about 9,900 years ago

I seriously don't get how how the data in (i) can lead to the answer of (ii)

The gorge of Niagara falls is erdoding upstream, not vertically. 1.3 metres by 9,900 years is about 12.87km. I am not familiar with the local geology, but would think that the beggining of the gorge is 12.87km from the current location of the falls, and that there have been various changes in the local are, such as the ice age ending, such that roughly 9,900 years ago, things changed so that a large amount of water found that the best way to go to the sea was over the edge of the plateau of rock that the gorge is carved through. Hence it gathered at the edge, started eroding and formed the gorge.

I am afraid this is probably better explained with a picture of the falls and surrounding area, but I dont have one. You could try googling for more info.

The gorge of Niagara falls is erdoding upstream, not vertically. 1.3 metres by 9,900 years is about 12.87km. I am not familiar with the local geology, but would think that the beggining of the gorge is 12.87km from the current location of the falls, and that there have been various changes in the local are, such as the ice age ending, such that roughly 9,900 years ago, things changed so that a large amount of water found that the best way to go to the sea was over the edge of the plateau of rock that the gorge is carved through. Hence it gathered at the edge, started eroding and formed the gorge.

I am afraid this is probably better explained with a picture of the falls and surrounding area, but I dont have one. You could try googling for more info.
Sorry, I don't get what you have said... :rolleyes:
What does "upstream" mean? water flowing upwards?

the begining of the gorge is 12.87km from the current location of the falls
Do you mean that the edge (top) of the fall was 12.87 km higher, lower, more inward, or more outward than its present location? Which part is being eroded at an average rate of 1.3 m/year?

http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Niagara_watervallen_canada.jpg/800px-Niagara_watervallen_canada.jpg

Quick question for my own curiosity- English is not your native language, or else you are only a young teenager, which is correct?

Upstream means further up the flow of water, so in that photo you posted, you are going upstream when you go towards the top left of the picture.
And its 12.87km more outwards, again, looking at that photo you have linked to, say, for illustrations sake, 500 years ago, the waterfall was more towards the bottom left of the photo, because it had not eroded all the rock below itself. Where you see the river now, was where the waterfall was 500, 2,000 or 9,000 years ago.

Quick question for my own curiosity- English is not your native language, or else you are only a young teenager, which is correct?

Upstream means further up the flow of water, so in that photo you posted, you are going upstream when you go towards the top left of the picture.
And its 12.87km more outwards, again, looking at that photo you have linked to, say, for illustrations sake, 500 years ago, the waterfall was more towards the bottom left of the photo, because it had not eroded all the rock below itself. Where you see the river now, was where the waterfall was 500, 2,000 or 9,000 years ago.
I am a Canadian and I am 16, is that young? :eek:

The word "upstream" means flowing against the current, right? For example, "A boat is moving upstream" means it is moving against the current, like going up the river. That's my interpretation of this word.

However, I am not sure what you mean by "The gorge of Niagara falls is erdoding upstream", which part is getting eroded? But I guess you are referring to the portion of the river with water that has not been cascaded down...and all the water that has been cascaded down is called downstream...am I getting this right?

Back to the question! If the fall is 12.87 km more towards the bottom left of my picture, that's really a great distance. But given that, how do they know that "the falls probably formed about 9,900 years ago" How do they know that point (12.87 km away) is the point where the fall starts forming?

No. Upstream means closer to the source. Downstream means further from the source. [Please don't ask me what the source is.]So, yes, if you are moving upstream you will be moving against the current, but that is incidental: it is not part of the definition.

The edge of the falls is undercut by the force of the falling water and its entrained solids. Eventually a piece falls off. That is erosion. The erosion is moving upstream. i.e. towards the source of the water.
The gorge starts 12.87 kms away. Beyond that there is no gorge. Go to your library. Consult detailed maps of the Niagara area. You will see what we mean.

I am a Canadian and I am 16, is that young? :eek:

The word "upstream" means flowing against the current, right? For example, "A boat is moving upstream" means it is moving against the current, like going up the river. That's my interpretation of this word.
Ahhh, well your age explains why you dont quite know what upstream means. And yes, going upstream is a matter of going in the opposite direction to the current.


However, I am not sure what you mean by "The gorge of Niagara falls is erdoding upstream", which part is getting eroded?
The rock underneath the river. Do you have any streams near you? (OK, theyre probably buried by snow) Go look at the layers of soil and rock on the banks of the streams, these have been eroded away by the stream. It is that material that is also getting eroded by the Niagara.


Back to the question! If the fall is 12.87 km more towards the bottom left of my picture, that's really a great distance. But given that, how do they know that "the falls probably formed about 9,900 years ago" How do they know that point (12.87 km away) is the point where the fall starts forming?
Because that is where the geology changes into something else. I am afraid that this is much easier to consider using geological maps or models, go ask your geology teacher at school.