Ruth and Simon
From your description and the photo I believe it is a Salt marsh caterpillar. Virginian tiger moth caterpillar looks similar but does not have the black face.
Larvae feed on a wide variety of mainly weedy plants including pigweed, anglepod , Sicklepod , Dog Fennel, ground cherry, and mallow, plus crops such as alfalfa, asparagus, bean, beet, cabbage, carrot, celery, clover, corn, cotton, lettuce, onion, pea, potato, soybean, sugarbeet, tobacco, tomato, and turnip.
This insect overwinters as a mature larva which pupates in early spring on the soil among leaf debris; adults emerge about 2 weeks later. Larvae are active, often found wandering over the soil in search of suitable food.
Here are some photos of adults and larva
http://bugguide.net/node/view/3242/bgimage
