This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The Quaternary is the last of the great divisions of geological time and may be said to be still in progress, for its events led by gradual steps to the present climatic and geographical order of things, and to the present geographical distribution of animals and plants over the surface of the earth. Quaternary deposits are to a very large extent continental in their origin, marine sediments in most regions being of very subordinate extent, and consist generally of loose, uncompacted sands, gravels, boulder clays, clays, and the like. These deposits never reach any very great thickness, but their horizontal extent is at least equal to that attained by any preceding system, for in one form or other they cover almost the entire surface of the globe. In an even greater degree than in the Tertiary are the Quaternary formations of different areas difficult to correlate, because of the locally restricted character of many of them, the frequent and radical changes of facies from point to point, and the scantiness of fossils or their absence over wide regions.
The line of division between the Tertiary and Quaternary is not easy to draw, especially in those regions which were not reached by the great glaciers and ice-sheets of the Pleistocene.
The seas at the end of the Pliocene had much the same extent as later, and on the same coasts the same kinds of material continued to accumulate, and some of the Pliocene coral reefs continued to grow uninterruptedly into the Pleistocene. Even in the regions of glaciation, the end of the Tertiary is fixed differently by different authorities.
According to general practice, the Quaternary is divided into two epochs, (1) the Pleistocene, and (2) the Recent, though various names are used for them.