I think you CAN see ice polished rocks still there.
I get to NYC pretty regularly and when I go I usually make it to Central Park,one of my favorite spots in the city.As you may know there are more than a few rocky outcrops scattered around the park.Those outcrops tend to feature weird looking (weird to *me*,at least) grooves and channels on top.The documentary I saw said that those groves were caused by the glacier slowly moving over the rocks and creating the grooves with rocks embedded in the ice itself.IOW,rock scraping against rock.
That's my understanding at least.