Having seen the feature marked on the map and then encountering a broad expanse of open rock I made the connection between the two based on my understanding of a piazza as an open space used as a meeting place. It transpires that Titan's Piazza is actually a formation of basalt lava columns further down the cliff face. I will have to go explore this feature sometime soon.
I also found an excellent web site devoted to the Holyoke Range with much historical information that can be found here.