Critical Reasoning

The Gulches is an area of volcanic rock that is gashed by many channels that lead downhill from the site of a prehistoric glacier to a river. The channels clearly were cut by running water. It was once accepted as fact that the cutting occurred gradually, as the glacier melted. But one geologist theorized that the channels were cut in a short time by an enormous flood. The channels do show physical evidence of having been formed quickly, but the flood theory was originally rejected because scientists knew of no natural process that could melt so much ice so quickly. Paradoxically, today the scientific community accepts the flood theory even though scientists still do not know of a process that can melt so much ice so quickly.

Which one of the following, if true, most helps to resolve the apparent paradox in the passage?

(A) Rippies, which indicate that the channels were cut by water, have been discovered in the floors of the channels.

(B) The Guiches is known to be similar in certain respects to many other volcanic rock formations.

(C) More than one glacier was present in the area during prehistoric times.

(D) Volcanic rock is more easily cut by water than are other forms of rock.

(E) Scientists now believe that the prehistoric glacier dammed a source of water, created a huge lake in the process, and then retreated.

jamesk486 wrote:The Gulches is an area of volcanic rock that is gashed by many channels that lead downhill from the site of a prehistoric glacier to a river. The channels clearly were cut by running water. It was once accepted as fact that the cutting occurred gradually, as the glacier melted. But one geologist theorized that the channels were cut in a short time by an enormous flood. The channels do show physical evidence of having been formed quickly, but the flood theory was originally rejected because scientists knew of no natural process that could melt so much ice so quickly. Paradoxically, today the scientific community accepts the flood theory even though scientists still do not know of a process that can melt so much ice so quickly.

Which one of the following, if true, most helps to resolve the apparent paradox in the passage?

(A) Rippies, which indicate that the channels were cut by water, have been discovered in the floors of the channels.

(B) The Guiches is known to be similar in certain respects to many other volcanic rock formations.

(C) More than one glacier was present in the area during prehistoric times.

(D) Volcanic rock is more easily cut by water than are other forms of rock.

(E) Scientists now believe that the prehistoric glacier dammed a source of water, created a huge lake in the process, and then retreated.

I'm split between C and E

C explains that there were more than 1 glaciers and so multiple individual
channels could have caused the cut

E gives a good theory that could have caused the cutting.

Since the question is mostly around a single glacier, I'm inclined towards
E.

OA please.

OA is E

OA is E

Can someone please explain why option (C) is wrong?

Premise: Scientists now accept a flood theory even though they don't know any process that can melt so much ice so quickly. OK.

But what if there were more than one glacier in the area that contributed to the creation of channels (as is indicated by option (C))? More glaciers means more water. More water means that in the same amount of time the melted water would do the same work as the water from one glacier over a longer period of time. Isn't that obvious? And, therefore, the flood theory holds, although they still don't know would could melt the water so quickly.

Can someone please explain why option (E) is the correct one?

Premise: The channels do show physical evidence of having been formed quickly...

Now, I don't see how damming the source of water and creating a huge lake in the process can quickly form the channels.

Thank you.

I received a PM for this one..

This is an LSAT question from 1994, the February test, the 2nd Logical Reasoning section, question 17.

Did you notice the jumbo-sized stimulus here?? This is a like a short reading passage! The reason is that this is a type of LSAT stimulus that is followed by TWO questions This is the second of those questions. I will post the other question for this stimulus after I have addressed this one. I just wanted everyone to rest assured that this is a long stimulus and probably longer than most that you would ever see on the GMAT.

Good thoughts from Jayhawk2001 up above (rock chalk jayhawk?)

Now we can see from the question that we are trying to explain the release of a flood of water so vast in such a short amount of time. And we know that the ice could not have melted quickly enough to cause the flood to be so violent.

The reason that choice E is the correct answer is that if the lake formed behind a dam of ice and then the ice melted enough that the dam was weakened then the water would break through and the entire dam is released at one time. So the channels are not formed when the dam is created and the water backs up into the lake but rather when the dam is breached because the ice that forms the dam has melted then the water is released all at once.

I have actually heard of this place. It is in either Eastern Washington state or else in Eastern Oregon. The area is pretty dry today but the gulches and ripples are really visible. The ice dam formed between two mountains and damned the river than had flowed through the valley. The lake that was created was huge! The size of many countries in Europe. The flood would have been incredible to watch.

Anyway, you do not have to know these facts. You just have to understand that you are looking to explain the fact that "today the scientific community accepts the flood theory even though scientists still do not know of a process that can melt so much ice so quickly." So you see this is where C is taken out. Even if you are talking multiple glaciers some how look at the wording of the paradox here "do not know of a process that can melt so much ice so quickly." One glacier or multiple glaciers does not matter. The water contained in that flood would take years to melt, not hours. This is why another explanation is needed. Option E is the only one that gives an alternate cause so to speak.

I just looked it up. The lake that formed behind the ice dam was glacial lake Missoula and the lake was in Montana and Idaho. The rushing water from the broken ice dam impacted both Eastern Washington and Eastern Oregon. American Public Television (PBS) did a special that the test writers must have seen.

Anyway here is the other question that goes with that stimulus this is #16 from that 2nd Logical Reasoning section, February 1994. OA in a bit give it a try first.

The Gulches is an area of volcanic rock that is gashed by many channels that lead downhill from the site of a prehistoric glacier to a river. The channels clearly were cut by running water. It was once accepted as fact that the cutting occurred gradually, as the glacier melted. But one geologist theorized that the channels were cut in a short time by an enormous flood. The channels do show physical evidence of having been formed quickly, but the flood theory was originally rejected because scientists knew of no natural process that could melt so much ice so quickly. Paradoxically, today the scientific community accepts the flood theory even though scientists still do not know of a process that can melt so much ice so quickly.

Which one of the following is supported by the information in the passage?

(A) Only running water can cause deep channels in volcanic rock.

(B) The river did not exist before the channels were cut.

(C) Geologists cannot determine the amount of heat required to melt a glacier quickly.

(D) The physical effects of water on rock vary with the speed with which those effects are
produced.

(E) Geologists are compelled to reject physical evidence when it leads to an unexplainable
conclusion.


OA in a bit give it a try first.

I would pick D.

A) Extreme. Stimulus says the Gulches were cut by running water. Nowhere does it allude that only running water can create such cuts.

B) This actually seems opposite to the information in the stimulus. If the river did not exist before the channels were cut, how were the channels cut? The premise states the channels were cut by running water.

C) The stem actually suggests the opposite. "scientists knew of no natural process that could melt so much ice so quickly." - meaning they must have known what kind of processes would melt the ice quickly - implying that they must have known how much heat would have been required to melt the ice.

D) "The channels do show physical evidence of having been formed quickly" - D follow from this.

E) Seems extreme. The geologists did not actually reject evidence, rather they have not been able to find enough evidence to prove that there was an enormous flood.

By the way... Crimson and Blue - Rock Chalk Jayhawk indeed

D for me too.

David, thanks for your reply. I think it's clear now.