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.
Fossil Fishing
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.
The shale accumulated as mud on a Triassic lake bottom, and it was covered by a coarse stream-laid gravel which has since been cemented into rock. The mud was not eroded by the stream which washed down the gravel, and the pebbles are not even impressed into the underlying shale. Apparently the fish perished as the waters evaporated and the lake became a playa flat. The limited variety of fish suggests that the connections with outside regions were restricted, and that living conditions within the basin were rigorous. The situation may have been like that found in the fresh water lakes along the western margin of the Great Basin in Nevada and eastern California.
Other lake deposits with fish remains appear at different levels from Whittemore’s Ferry up to the Sunderland Caves. Each is covered by a conglomerate layer, and at each place the lake clays had been partially hardened before the pebbles were washed over them. Seemingly dry alluvial plains followed transient lakes in kaleidoscopic but cyclic succession.
Calendar Beds
The lakes in which the fish lived and died date back to late Triassic time. Much younger were the lakes that followed the continental ice sheets, and in many valley localities these younger water bodies have registered their brief span of geologic life. For they, too, were settling basins for clays, which are characterized by annual depositional bands like the growth rings in a tree. These clays may be examined best in the clay pits at any of the brickyards, particularly at South Hadley Falls, or beside the high river banks rising above the Connecticut flood plain just south of Hadley.
The clays consist of alternating thin, dark, fine bands and thick, light, coarser ones. The coarser bands are sandy, and some of them have ripple-marks. The total number of pairs of beds is the number of years that glacial Lakes Springfield and Hadley inundated the valley, but it is not a simple matter to count them. Actually the lake bottom deposits are but a small fraction of the total volume of material brought to the lake. Lake shore deposits and deltas grew outward and buried the bottom deposits after a few hundred years had passed. Thus in the pits at South Hadley Falls, the clays rest upon glacial gravels, and a scant hundred layers intervene between them and the sands above. Shore encroachment is not encountered at Hadley, but the shallow depth of the present water table hinders deep exploration, and the Fort River has removed many of the upper beds.