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Updated: June 21, 2025
The leading varieties of the trappean rocks basalt, greenstone, trachyte, and the rest are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless masses protruding through or overlying them, or in horizontal sheets intercalated between strata.
All strictly glacial deposits are unstratified. The load laid down at the end of a glacier in the terminal moraine is loose in texture, while the drift lodged beneath the glacier as ground moraine is often an extremely dense, stony clay, having been compacted under the pressure of the overriding ice. EROSION. A glacier erodes its bed and banks in two ways, by abrasion and by plucking.
When a glacier, thus laden, descends so far as to reach a region about 3500 feet above the level of the sea, the warmth of the air is such that it melts rapidly in summer, and all the mud, sand, and pieces of rock are slowly deposited at its lower end, forming a confused heap of unstratified rubbish called a MORAINE, and resembling the TILL before described.
The brunt of the controversy settled about the unstratified rocks granites and their allies which the Plutonists claimed as of igneous origin. This contention had the theoretical support of the nebular hypothesis, then gaining ground, which supposed the earth to be a cooling globe.
Over large areas it is spread in level plains of till, perhaps bowlder- dotted, similar to the plains of stony clay left in Spitzbergen by the recent retreat of some of the glaciers of that island. In places the unstratified drift is heaped in hills of various kinds, which we will now describe.
Sometimes, however, it is a soft clay, at others it is mixed with sand, but whatever the composition of the underclays may be, they always agree in being unstratified. They also agree in this respect that the peculiar fossils known as stigmariae abound in them, and in some cases to such an extent that the clay is one thickly-matted mass of the filamentous rootlets of these fossils.
The material of the dump is for the most part unsorted, though the water of the melting ice may find opportunity to leave patches of stratified sands and gravels in the midst of the unstratified mass of drift, and the finer material is in places washed away. The stream is gray with silt and loaded with sand and gravel washed from the ground moraine.
A simpler term, however, quite as distinctive, and more descriptive of their structure, is that of the stratified and unstratified or massive rocks.
For these deposits cannot originally have ended abruptly as they now do, but must have once been continuous farther towards the centre of the valley. Brown clay with angular flints, and occasionally Chalk rubble, unstratified, following the slope of the hill, probably of subaerial origin, of very varying thickness, from 2 to 5 feet and upwards. 2.
Fragments of rock have travelled for great distances, especially from north to south: the surface of the subjacent rock is smoothed, striated, and fluted; unstratified mud or TILL containing boulders is associated with strata of loam, sand, and clay, usually devoid of fossils.
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