#8 - Must Be True

[Administrator]

Complete Question Explanation
(See the complete passage discussion here: lsat/viewtopic.php?t=14432)

The correct answer choice is (D)

This question asks us to identify an area (other than the dating of past earthquakes) to which
lichenometry would be particularly applicable. Recall that the process of using lichen shape and size
as clues to the location and frequency of earthquakes was described in the second paragraph. Using
that information, look for an answer choice describing an analogous situation where lichen size
would be of value in gauging the occurrence of some past event.

Answer choice (A): Recall that lichenometry involves measuring the size of lichens growing on
exposed rocks at a slow but constant rate if left undisturbed (lines 26-28). Since each flooding of the
river would disturb the normal growth of lichens in the flooded area, it is unlikely that lichenometry
can be used to identify the number of times a particular river has flooded. Lichenometry may help
identify the timing of the most recent flooding, but not the number of times the river has flooded
over the years.

Answer choice (B): Since lichens grow at a constant rate for no more than 1,000 years (line 28), it
is unlikely that we can use them to identify the age of a fossilized skeleton of a mammal that lived
thousands of years ago.

Answer choice (C): Given that lichenometry involves measuring the size of lichens growing on
exposed rocks, it is unclear how we can use it to identify the age of an ancient beach that is currently
underwater.

Answer choice (D): This is the correct answer. Since lichens grow radially at a constant rate, the
diameter of the largest lichen would provide direct evidence of the initial position of the glacier at
the bottom of the mountain valley. As the glacier recedes, the lichens would have less and less time
to develop and therefore decrease in size and abundance. Thus, if we know the rate of lichen growth,
we can also calculate the rate at which the glacier has been receding up the valley.

Answer choice (E): Although lichenometry is best used for earthquakes that occurred within the last
500 years, do not be misled by the analogous time frame in this answer choice. Given what we know
about lichenometry, it is unclear how this method can be used to identify trends in annual rainfall
rates, especially since the varying amounts of rainfall can potentially affect the growth rate of lichen
colonies in the valley.

[alee]

Hi guys,

I have a question about Q8 of Preptest 62, Section 1 (Passage 1). How do you distinguish between the correct answer (D):

(D) Identifying the rate, in kilometres per century, at which a glacier has been receding up a mountain valley

and the incorrect answer (E)

(E) Identifying local trends in annual rainfall rates in a particular valley over the past five centuries

I eliminated A, B and C immediately on the basis of lines 53-54 stating that lichenometry is best used for earthquakes that occurred within the last 500 years. Is the rationale for choosing D over E that lichenometry is a method based assessing the age of rocks left exposed (in the case of earthquakes, by rockfalls)? This implies that it's more useful in the glacier case, which as it recedes alongside a MOUNTAIN (i.e. rocks), would leave some rocks newly exposed, hence prompting lichen growth. There is no such 'leaving rocks newly exposed' linked with rainfall.

Is that the correct explanation?

Thanks!

[Adam Tyson]

That's correct, alee, but there's one additional reason to reject answer E, and that is that rainfall is not mentioned as a factor affecting lichen formation and growth in the passage. Wind and shade are mentioned, but not rain. Since the key factor in using lichenometry is that the lichens form on newly exposed rock, the glacier case is analogous to the rockfalls associated with earthquakes.

Adam M. Tyson
PowerScore LSAT Instructor

[moshei24]

Why is the answer (D)? How would the lichen tell us the rate it's been receding? It tells us how long it has been since it became a glacier (fell off the rest of the ice), but how could it tell us the rate?

Also, why is (C) wrong? It's referring to age, so what is the issue with that answer choice?

Thank you.

[Adam Tyson]

I think you may be confusing a glacier with an iceberg here, moshei! A glacier is a sheet of ice that moves over land, generally very slowly. Imagine the ground under the edge of a glacier as it recedes - rocks that had previously been under it become exposed, right? Lichens then start to grow at their slow but steady rate. We would expect that the oldest lichens would be at the further distance from the glacier's receding edge, and they would gradually become younger as we moved towards the glacier. Licenometry could be used to calculate the age of the lichens at various intervals, and thereby determine how quickly the glacier was moving over a given stretch of land. (For example, if the lichens at point A are 100 years old, and the lichens at point B, 1 mile away, are 90 years old, then the glacier must have taken 10 years to recede over that mile).

Get it?

Adam

[moshei24]

Oh, gotcha.

Thank you.

[snowy]

Hi! I got this question correct but just want to double check if my reasoning was sound, since it’s a little different than the original post by the administrator explaining each answer choice.

I ruled out A-C because they didn’t fit the 500-year window mention in line 54. Then I was between D and E, and ultimately went with D because lichenometry is used for things related to seismic shaking, and I thought glaciers have more to do with seismic shifts than rainfall does. Are those both correct ways of reasoning? Thank you!!

[Brook Miscoski]

Snowy,

I like your reasoning for eliminating (A-C), but I think your reasoning for choosing (D) should be improved.

The passage states that lichenometry is accurate to within 10 years. Thus, it would be useful for establishing a per-century change, but would not be capable of establishing a per-year change. Thus, (D) is correct, and (E) is incorrect.

If you think about it, this is just a continuation of the way that you eliminated (A-C), you just needed to keep going with the years approach.

[snowy]

Thanks so much, Brook! I didn’t even think of that implication of “annual” rainfall rates. That makes sense!