Dodge.J 260 [March 21, 
accident miglit be formed where a glacial front advancing into 
the headwaters of a river system thereby furnished a much 
larger h)ad than it would be possible for the stream to remove 
if its vohime remained constant. The effect would be an 
immediate overloading of the stre:im :ind consequently the 
formation of an alluvial plain whicli might be terraced Inter, 
when tlie glacier had retreated. The terraces in the liead waters 
of certain rivers of northwestern Pennsylvania seem to have 
been formed in this manner. 
CoJiPOsrrE Alluvial TEuuAtics. 
As I have already mentioned, composite terraces are those due 
to some terrace-making process which, beginning in one cycle, 
closes in a subsequent one, being thus the result of a continuous 
process acting in a river valley while the continent as a whole was 
rising or sinking. Consider for instance the case of a river which 
had reduced the greater part of its drainage area to base-level in 
one cycle and was wandering across an alluvial ])lain. If the 
land mass was then raised, the river would be I'ejnvenated and 
would at once begin to cut a channel in the i)reviously accumu- 
lated alluvial deposits. The terrace plain would thus have been 
formed in one cycle, the escarpment in the succeeding cycle ; and 
the terrace as a whole would be conii)osite in character. It seems 
probable, as I shall try to show in this paper later on, that manj^ 
of the famous terraces in the northern portion of the United 
States are composite in character. 
Complex ok Inv<)lvei> Alluvlvl Tekraces. 
Involved terraces, or as it seems better to call them, com])lex 
terraces are those due to a combination of causes acting jointly 
or in succession, and continuing through the whole or part of one 
or more geographical cycles. If we could know all the factors 
that have determined the formation of most of the terraces we see 
at the present day, T think tliat we should find them to be better 
classed as comjdex terraces than under any other heading in the 
classification thus far proposed. 
