In other cases small surveys among these fjords have shown that several of the larger islands are cut by channels which separate them into smaller ones, while elsewhere the low valleys which unite the mountains and hills are the result of post-Glacial deposits that have filled part of the former channels, these islands being the summits of an old continuous half-submerged mountain chain.
Superimposed on these rocks are Pleistocene boulder clay, and clay and sand deposited in post-glacial lakes or an extension of the Gulf of St Lawrence.
Conclusive proofs, however, of a later submergence under a post-Glacial Littorina sea (containing shells now living in the Baltic) are found up to 150 ft.
In post-Glacial times, a subsidence admitted the sea into the Lagan valley and across the eastern shore in several places; but elevation, in the days of early human occupation, brought these last marine deposits to light, and raised the beaches and shore-terraces some 10 to 20 ft.
The former of these, representing the bottommoraine of the ice-sheet, are covered with Glacial and post-Glacial clays (partly of lacustrine and partly of marine origin) only in the peripheral coast-region - or in separate areas in the interior depressions.
The deposits of the Post-Glacial period are represented throughout Russia, Poland and Finland, as also throughout Siberia and Central Asia, by very thick lacustrine deposits, which show that, after the melting of the ice-sheet, the country was covered with immense lakes, connected by broad channels (the fjarden of the Swedes), which later on gave rise to the actual rivers.
In former ages the tree covered a large portion of the more northern part of the island, as well as of Ireland; the numerous trunks found everywhere in the mosses and peat-bogs of the northern counties of England attest its abundance there in prehistoric times; and in the remoter post-Glacial epoch its range was probably vastly more extended.
Francis Chamberlain and Holmes that they are post-Glacial and comparatively recent (Am.
Recently elevated marine clays, of post-glacial date, fringe the south-eastern coast, while gravels with marine shells, side by side with flint implements chipped by early man, have been lifted some 20 ft.
This and other evidence (which is considered in more detail in the article Archaeology) is now generally accepted by geologists as carrying back the existence of man into the period of the post-glacial drift, in what is now called the Quaternary period, an antiquity at least of tens of thousands of years.