Shepherd's sheep up on sedimentary rock curtailed by Sierra (8)

I believe the answer is:

marshals

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'up on sedimentary rock curtailed by sierra' is the definition.
I can't tell whether this definition defines the answer.

'shepherd's sheep' is the wordplay.
'shepherd' becomes 'marshal' (both can mean to gather).
'sheep' becomes 's' (this could be a standard abbreviation of which I'm unaware).
'marshal'+'s'='MARSHALS'

Can you help me to learn more?

(Other definitions for marshals that I've seen before include "Chief officers" , "Arranges" , "Crowd-controllers at large public events" , "Officials" .)

I've seen this clue in The Irish Times.

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