This bit of info may be too late, but here goes anyway. There were Presbyterian churches located in both the community of "South Mountain" and the community that once was called "North Mountain" or just "Mountain". The church in South Mountain (I don't recall its name) has been closed for a number of years and converted to a private residence. I believe the church in the village of Mountain (Knox Presbyterian) still holds Sunday services.
Lynne Cooke a local genealogist and UEL descendant has amassed a large collection of information on Dundas Co. including various church records. Lynne lives in Morrisburg, Ont. and will let yu look through her library upon appointment. She has a web site where yu can get further information.
Hope this helps. Good luck.