in squantum, massachusetts, hoffman shows us features which reveal a partial ice age about 580 million years ago. hoffman: well, what we see here is a completely disorganized mixture of clay and silt and sand and stones and boulders that have all been churned and rubbed together due to glacial action. so, if you look underneath any active glacier, you see this bed of muck that's full of stones and boulders and whatnot that's getting dragged along by the flowing ice overtop, and it's a very distinctive kind of deposit. it looks like it just got dumped out of a cement mixer, okay, and that indicates that there were no waves or currents involved in the deposition of this material. this is something that was just churning everything together, and the only process of transport that operates in that way is transport beneath a flowing glacier. narrator: in addition to indicating the presence of glaciers, the rocks can also reveal when those glaciers started to recede and the planet started getting warmer. this is shown by laminations, or layers. so, what i'm looking for here is a layer of str