I crossed over to Canada at the Rainbow brdige, Niagara Falls. I presented my GC without the passport. I was not asked for it. I asked the immigration officer whether a paasport was necessary and he said no, not at land crossings from the US. On the return to USA it is clear that you do not need to present your passport, only your GC The USCIS website states this clearly. I did not ask the Customs and Border protection officer at the crossing on my renetry the next day as there was a huge line and he seemed to be in a rather grumpy mood.