Police station, motorway and lake at the bottom of unusual garden (11)

I believe the answer is:
gendarmerie
'police station' is the definition.
'gendarmerie' can be an answer for 'police' (I've seen this before). I'm unsure of the 'station' bit.
'motorway and lake at the bottom of unusual garden' is the wordplay.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'and' is a charade indicator (letters next to each other).
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'at the bottom of' says to put letters next to each other (in a down clue, some letters go below others).
'unusual' indicates an anagram.
'garden' anagrammed gives 'gendar'.
'm' put next to 'erie' is 'merie'.
'merie' put after 'gendar' is 'GENDARMERIE'.
Can you help me to learn more?
(Other definitions for gendarmerie that I've seen before include "French policing force" , "foreign force" , "Police force" , "French law keepers" , "French paramilitary police" .)
