220 The American Geologist. April, 1902. 
Sherborn, the river is closely bordered by the highlands for 
some distance. These are critical points in the history of the 
river; and of special significance are several gaps prominent 
in the watershed between the Charles and adjacent basins; as 
for instance, at Bellingham, Walpole and Cochituate. The geo- 
logical structure of the region is shown on plate 13. In this part 
of Massachusetts are three areas, or basins, of sedimentary 
rocks, the Boston basin on the northeast, the Narragansett basin 
on the south, and the Norfolk basin, a long narrow, connecting 
trough. These basins are occupied mostly by Carboniferous 
strata,—conglomerates, sandstones, slates, and in the Boston 
basin contemporaneous lavas,—which are in general less re. 
sistant than the surrounding granites, diorytes and felsytes. 
For this reason the Carboniferous areas are usually topographic 
as well as geologic basins. 
The history of the Charles river began in Tertiary time. 
During the preceding period marine erosion had developed 
the Cretaceous* peneplain, which was covered in all its sea- 
ward portion by Cretaceous sediments. At the close of the Cre- 
taceous period an elevation of the land took place, raising the 
peneplain with its burden of sediments out of the water. 
Across the new land surface thus formed, the streams must 
have taken the shortest courses to the sea, utterly regardless 
of the structure of the underlying pre-Cretaceous rocks. At 
first they flowed upon the unconsolidated Cretaceous sediments ; 
but as these were gradually removed by erosion, the hard rocks 
beneath were attacked, and the formation of a second, the Ter- 
tiary* peneplain commenced. 
At the time of the Cretaceous elevation the streams were 
original, and consequent upon the slope of the land. As, by con- 
tinued erosion, the Cretaceous sediments were slowly removed, 
the streams became superimposed upon the underlying form- 
ations. The basin of sedimentary rocks, being least resistant, 
were eroded more rapidly than the bordering crystallines; 
and the streams gradually shifted. their course on the softer 
rocks. Thus they became adjusted, during the Tertiary period, 
to run in the basins as much as possible. The streams rising 
*W. O. Crosny.—Geological History of the Nashua Valley During the Ter- 
tiary and Quaternary Periods, Technology Quarterly, vol. xii, No. 4, p. 289; 
Geology of the Boston Basin, Part 3, p. 538, Occasional Papers, Boston Soc. 
Nat. Hist. 
