a3 
The Table of Geologic Time 
mg! 
The history of any region prior to events observed by man is 
pieced together from records left by processes of nature in its geo- 
logic formations and landforms. No single area contains a complete 
record since earliest geologic time. At a place like the Grand Can- 
yon in Arizona there are exposed vast thicknesses of rock layers 
that lie like a pile of manuscript pages, carrying the story of mil- 
lions of years of earth history. Yet even here there are gaps in 
the records; pages are missing from the manuscript, because at some 
early date erosion gnawed at the top of the pile for a time and car- 
ried off whole chapters before later ones were written. Such inter- 
ludes may be more fully recorded ina different locality, however, 
ard diligent search is necessary to complete the chronicle. 
Where sediments were laid down in layers and were not later 
overturned by some great earth movement, it follows that the sequence 
of the layers gives the sequence of events or- conditions in their 
‘formation. It is possible, too, that layers in one part of an area 
were contemporaneous with those in another as,for-example, the wind- 
blowvm sards of “the Cape Cod dunes are contemporaneous with the 
beaches and bottom muds. now forming in Buzzards Bay. 
Periodically throughout the past the continuous flow of time 
has been.. punctuated by widespread- upheavals of the earth. These 
caused interruptions in depositional processes and made possible the 
recocrition of.-the units of geologic time, listed in what is known 
as the table of geolozic time’. It encompasses more than a billion 
years. (See Table 1). Its longer subdivisions are known as eras. 
They consist of périods which, in turn, may be separated into epochs. 
Numerous smaller-divisions are discovered as geologic studies con- 
tinue to unravel the complicated story of the earth's past. 
In contrast to the Grand Canyon, Cape Cod has .long been known 
to corsist of only a relatively thin veneer of sediments formed in 
this part of the. world during a very short: and much more recenti/ 
1/ Note that this use of the word, recent, is not the same as its 
techrical use in the time-table. In this report it will be capital- 
ized wnerever the restricted time interval of the table is intended. 
episods, so that they are not yet consolidated into rock and may, 
indeed, be carried away by wave and stream erosion before consolida- 
tion can occure Firmly consolidated rocks are not exposed anywhere 
on the Cape, but are believed .to lie several hundred feet below the 
surface rear Woods Hole, and may be nearly a thousand feet below at 
Provincetowne This report, therefore, is concerned with only the 
upper more recent part of the geologists! time-table, comprising tut 
a fraction of the last million years of world history. Consequently 
all of the earlier, lower.part of the accompanying time-table has 
been simplified and abbreviated, 
