Peggy
Yes, you do have a sphinx caterpillar! This genus does have rather smooth caterpillars. There are several that look similar and without actually seeing the caterpillar I am not positive which one you have.
I am guessing you have either the Northern Apple sphinx -http://bugguide.net/node/view/41976/bgimage?from=24
Or the Clemens' sphinx - http://bugguide.net/node/view/24902/bgimage
These are both found in more northern parts of North America. These caterpillars both have the black stripe on the tail and a green head.
In both cases there is just one generation per year and it overwinters as a pupa in the soil. Moths of both feed on nectar.
Caterpillars of the Clements feed on apple, ash, northern bayberry, birch, poplar, willows and some other plants.
Caterpillars of the Northern Apple feed on leaves of apple, blueberry, meadowsweet , spruce, Sweet Gale, and tamarack. The Northern apple caterpillar is very closely related to the Apple sphinx...some put them as one species, so that is another possible name of your caterpillar :-).
Perhaps knowing the plants it feeds on will help with ID? If you know what plants were in that area?
