It seems like I'm seeing worm castings everywhere I go these days, like these I photographed this morning on the trail in the Back Five.
According to Life in the Soil, identifying different earthworm species involves characteristics such as the number of segments between the clitellum and the head of the earthworm, the number of segments the clitellum takes up, and the shape of the first head segment. No, I haven't attempted this yet! (But I probably will, if I can find a decent guide....)
Earthworms will dig as deep as 10 feet, and they literally eat the soil as they push their way through it. The resulting manure - the earthworm's castings - is both less acidic and richer than the soil the earthworm took in. Calcium carbonate has been secreted into the material by glands along the earthworm's gut, and the resultant castings therefore have about 50% more available nitrogen, phosphorous, potassium and calcium than the surrounding soil.
Earthworms move a surprising amount of soil. Nardi reports that Darwin's last book was about earthworms, and that Darwin estimated that earthworms produced about 40 tons of castings/acre/year on the soil surface in Britain. He also estimated that this activity would cause objects to be buried at a rate of 0.1"/year simply by the actions of earthworms!
I guess the earth really is moving under our feet when there are plenty of earthworms in it!