296 REPORT OF THE AMERICAN COMMITTEE. 
The Lower and Upper Drift series are more or less alike in 
comjKwition, but they are in many places separated by soils hold- 
ing the remains of rich vegetation. Each of the series of the 
Drift is of itself of complex structure, with even intercalated 
layers liolding plant remains. The Drift is most commonly 
composed of clays often containing glaciated stones with various 
admixtures of sand, or sometimes considerable quantities of 
boulders. A minor proportion of these materials has been trans- 
ported from a distance. These deposits often show no^ or only 
obscure, stratification ; yet this structure may be developed, or it 
may be present in the form of included beds of clay or sand. 
The Lower Drift is in the form of irregular sheets, which may 
be regarded as composed of two separated series, above the upper 
of Avhicli the Loess of the Mississippi has been placed. This ob- 
scurely stratified, or not stratified, fine, homogeneous, calcareous 
sandy clay contains land and fresh water shells and bones of 
mammals. The Upper Drift is more commonly in the form of 
moraine-like ridges, of great extent, developed in lobes, especially 
in front of the Great Lakes. However, these ridges are apt to be 
capped by more or less irregularly stratified deposits of clay, sand 
or gravel. Such hills form many of the well-known " Hog- 
backs " of the Drift region. 
The Erie clay is best represented in the Lake region, and is a 
well stratified, generally stoneless blue clay, derived from the 
older Drift. It also includes some stony stratified clays. Its 
surface is extensively denuded, and upon it rests the more sandy 
yellow and brown Saugeen clays (a name including a group of 
clays). These stratified deposits of the interior of the Continent 
are probably the equivalents in part of the more eastern marine 
Champlain beds. However, their exact relationship has not been 
made out. Some of the beds included here are doubtless the 
deeper lacustrine equivalents of some of the beaches of the next 
series. 
The Terrace and Beach series embrace a large group. Some 
of the terraces date back to the earlier Plistocene period, and are 
represented by sea-cliffs cut out of the older rocks. Newer 
terraces are cut in the softer materials of the Plistocene beds. 
There are also terraces of construction and numerous beaches of 
sand and gravel. The terraces are most strikingly developed in 
the river valleys ; the beaches (with associated terraces) about the 
