During an ice age with mile thick ice over Seattle, I’m guessing that there would be opportunity for that ice to break and calve off like it does today. When a 100-foot chunk breaks off in some Alaska bay it will cause a water rise. One minute you are standing in the water with your rubber boots, the next the water is suddenly pouring over the tops.
Having a chunk of ice thousands of feet wide and thousands of feet tall break off would have been fairly common 15,000 years ago I would think. I’m not sure what the dimensions of that island out in the east Atlantic is - but there are articles written on the huge tsunami that it would cause if there was a huge landslide.
And landslides/ice falls can cause larger tsunamis than a large earthquake.
The tsunamis mentioned in replies are localized in effect. Glacier drops/breaking dams are catastrophic regionally, but not to the extent of creating a tsunami that travels thousands of miles to inundate tropical islands, which are the subject of the original post.
Those islands sprout along the pacific rim and are susceptible to tsunamis from earthquakes. Glacier issues would never have troubled them.
Read the book. It is well worth the time. It didn’t “calve off.” Many cubic kilometers sluiced out through the Hudson bay in one gush. That was ice on land and would raise the sea level by itself fairly rapidly as well as creating the tsunami. It was not a slow regular process and the huge glacier lake pouring out behind it amplified the effect. Very strong earthquakes accompanied the change in pressure on the land and volcanism increased. That is why every old culture has flood myths, especially in the SEA area where the greatest expanse of land was subsumed in three episodes between 11000 years ago and 7500 years ago. The sea level is still rising a few cm a decade from those events. The AGWists do have some inkling of what is happening but no notion at all of why and they think they can godlike stem the tide if only we give them enough money and power.