We have a bit of a geological mystery on our hands… Everywhere we dig we find rather large round rocks – about the size of a basketball (+/-) and quite smooth…

They’re in the back yard. They’re in the cellar (when we dug the trench for the new drain pipes). They seem to be everywhere. They look like river rocks – from a fairly large stream or a small river. But there’s no apparent sign of wetness.

We’ve seen townhouses in Harlem that have streams running under them (one in particular on 141st – on the edge of Hamilton Heights – across the street from the new location of Alexander Hamilton’s Grange) – but there’s no sign of a stream at our place.

So how did so many river rocks get on our property? Could these have been left by glaciers? Are they common in Harlem or in Manhattan in general?