How Did These Rocks Get Here?

In front of my porch, I have a small planting area for flowers.  Ringing the planting area is a row of rocks, some of which may have been found in the yard, but many of which likely weren't.  Now, it's been quite a while since I learned how to identify rocks, but I know that most of the bedrock around here is composed of Devonian shale (the famous Marcellus Shale), a sedimentary rock that formed when central New York was the bed of a shallow inland sea.  But of the rocks in front of my porch, many of them look to be metamorphic, with embedded specks of darker material or ripples or waves of color, as in the rock pictured above.  One of them is a red sandstone...there's nothing like that around here.  My guess is that a previous owner of the property was something of a rock hound, collecting unique rocks from across the area.  These metamorphic rocks, and maybe even the sandstone, may be found in central New York, but they would have been transported here by glaciers...they're known as glacial erratics.  The fact that they were carried by glaciers, then likely deposited in runoff channels, explains their highly smoothed appearance.  It might be neat, once the snow clears up and I get a good look at them again, to find out exactly what kind of rocks they are and where they might have come from.