Simple past Vs Past Perfect - Once and for all..

[sunnymalhi]

His studies of ice-polished rocks in his Alpine homeland, far outside the range of present day glaciers,led Agassiz in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.

a) in which great ice sheets existed in areas that are temperate now.
b) in which great ice sheets had existed in areas that are temperate now.

Which of the two ?
More than the answer, am looking at the difference b/w the two constructions...When do we use "had"...? Another similar sentence may be "He messaged me" or " He had messaged me". Whats the semantic difference?

Appreciate your help.. thanks.

[prachipareekh]

If an action began in the past and continues into the present, use the present perfect tense.(Present Perfect = HAVE/ HAS + Past participle).

If an action precedes an earlier past action, use the past perfect tense.(Past Perfect = HAD + Past Participle)

Otherwise, stick to simple tenses - as GMAT prefers simplicity.
e.g -

Incorrect - I think that ancient people HAD BELIEVED in many Gods.
Correct - I think that ancient people BELIEVED in many Gods.

In the above sentence the past perfect (had believed) is unnecessary because the sentence involves only one action in the past tense. Therefore, the simple past (believed) is correct.

Note the difference in meanings in the two sentences stated below

When I switched the TV on, the programme started. I was just in time.
When I switched the TV on, the programme had started. I missed the beginning.

Following the same analogy my answer choice -- (a)

[amitg_ind]

Agree with Prache.
If there are two events in past to compare the time, use past perfect, but in here, there is only ONE. Hence SIMPLE PAST would do.

[maniranjank]

He had messaged me as a sentence is incorrect. Instead He messaged me should be used. The difference is, as mentioned in prachipareekh's response, about the time of action in the past. had messaged will be appropriate in a sentence where there is another action is the past and messaging preceded that action. A sample sentence is I regretted that I had lost my temper.

in which great ice sheets had existed conveys the meaning that great ice ceased to exist sometime in the past which is against the intended meaning.