The Era of Modern Life

The next era, the Tertiary, though it began some sixty million years ago, is described as ‘modern’ by the geologist, since, during its course, many forms of life known to us began to appear. In New Zealand it commenced with a gradual advance of the sea over the old land-surface of Mesozoic times. The spoil then carried down from uplands formed coarse-grained grits, conglomerates, and sandstones. These were succeeded by coal-measures similar in origin to those of Cretaceous times. As the sea advanced farther, shallow-water greensands were laid down, and finally accumulations of the hard parts of sea animals were built up in water too far removed from the land to contain sand or mud. These are termed limestones, and they are so widespread in Middle Tertiary formations that very little land could have remained above the sea.

That some land did survive, however, is proved by fresh-water deposits containing impressions of plants. These are quite modern in type, showing that flowering plants had now replaced the fern-like and conifer plants of the Mesozoic Era.

After Middle Tertiary times the downward movement of the New Zealand area was reversed, and the land began to rise. Limestone continued to be formed, particularly in parts of the North Island, but as the waters became shallower, there gradually appeared different deposits. These were fine-grained muds and sands washed into the sea from the emerging land.

The uplift of most of the South Island ended marine sedimentation, but in Marlborough and over much of the North Island it continued to later date. The spoil worn down from the granite mountains of Nelson and northern Westland was carried north by ocean currents and spread over Taranaki, Wellington, and Hawke’s Bay to form the widespread blue mudstones, or ‘ papa,’ which are the youngest Tertiary strata in those areas.

Throughout   the Tertiary   Era   there   was   a gradual change in climate, a change that was to culminate in a severe ice age in the next era. In early and Middle Tertiary times, however, the climate in New Zealand was much warmer than at present,  and in the shallow seas which flooded there flourished a rich and varied animal life, including some forms now found only in warm-temperature and tropical climates. From sands, limestones and mudstones formed in these times, therefore many well-preserved fossils   can be collected. The specimens from the earliest strata belongs to extinct kinds, but as we pass to higher strata, there is a gradual increase in the number of species which are identical with the life in New Zealand seas to-day.

The fossils of the marine beds belong to many groups. The lowly, single-celled Foraminifera, found in great numbers, are of great value to geologists when seeking for petroleum in com­mercial quantities. The spicules, which form the framework of sponges, are numerous in the famous chalk-like deposits of the Oamaru district. These also contain exquisitely constructed primitive plants called diatoms and lowly animals known as radiolaria. Among the larger fossils the most important are shells of many varieties, while corals, barnacles, and crabs are found in smaller numbers. Vertebrates (animals with a spinal column) are rare, but occasionally fragments of fossil reptiles and whales have been discovered. Fossil bones of a gigantic penguin are known, and sharks’ teeth are comparatively common. Altogether, these Tertiary deposits provide a wonderful hunting-ground for the geologist, amateur or professional.