The large tracks invariably have impressions of three toes. Even a careful search does not disclose the double tracks which would have been left by quadrupeds, and for years the bipedal impressions were called bird tracks. But birds have spurs which leave a mark behind the middle toe; these animals had no spurs and were not birds, but reptiles. Gregarious animals generally follow a leader, and only an occasional individual strays from the beaten path. The tracks at Holyoke suggest that these Triassic reptiles traveled in small herds.
The modern silts of the Connecticut Valley are not a good medium for the preservation of tracks because they lack coherence, and they drift with the wind as soon as they dry. Clays in a region of seasonal aridity are different. They are baked hard in the hot sun, and the water contains dissolved mineral matter which crystallizes in the clay and sand as the water evaporates, cementing the particles into a rock-like aggregate. Impressions in this sort of mud are preserved. The Connecticut Valley had the right kind of sediment and climate in Triassic time; impressions of salt crystals can be found in the shales where the tracks are clearest, not only in this locality but elsewhere in the neighborhood of Holyoke and West Springfield. These precipitated salts helped hold the clays together until they were effectively buried, and afterwards a firmer cement was deposited around the particles.
Footprints are known near South Hadley, at Turners Falls, at Gill, and along the highway to the French King Bridge; but they do not portray the character of the animals, their habits and the mode of preservation of their tracks as effectively as the tracks north of Holyoke. Certainly no occurrence of tracks _in situ_ is as accessible, and no geological exhibit in New England has received so many visitors.
Many years ago men were excavating to lay a foundation for a waterwheel at what was Whittemore's Ferry, three miles north of Sunderland. They made a catch of some of the most ancient fish ever taken in New England, but the fish were petrified and did not put up a fight.
They were found in layers of black shale, in which skeletons and carbonized tissues were well preserved. Of the five genera identified, all but one were ganoids.