John, you are right and kbartleson is wrong in that it was only the surface layer of an ice sheet over 2 km thick that melted. But saying the cycle of these melts is 150 years is kind of misleading. The gap between the 1889 event and the one prior to it was 700 years. These surface melt events were concentrated in the first half of the Holocene and have been relatively rare during the last 1000 years. This graph:
http://www.gisp2.sr.unh.edu/DATA/alley1.htmlshows there have been only 6 events in the last 2000 years, an average of every 300 years and 3 of the events were concentrated during the MWP. The frequency of the events in the early Holocene is probably due to orbital variations that don't exist today.
