Global warming!!!
So, 1300 years ago there was no glacier there. Sounds like we’ve been in a global cooling period, and the glaciers have now begun to retreat.
Some retreat, some advance. Thankfully, so far we have not started the nest ice age. I always get a kick out of scientific reports about Antarctica having a tropical climate at one time. Water covered Florida in the past, the Mediterranean Sea was probably farmland, and the Sahara had a sea.
Climate always changes, with the cycle of the Sun.