226 The American Geologist. April, 1902. 
bridge deep borings have been made which lie exactly in line 
with this gorge; and with a slight bend, the surface depression 
extends with an average width of two miles, across Belmont 
and Arlington to West Medford. Professor Crosby has 
shown? that the Merrimac river in pre-Glacial time was prob- 
ably tributary to Boston bay. If such was the case, its mpst 
direct course would be directly southward from Arlington 
along this depression to the Old Harbor, which would make 
the preglacial Charles a branch of the Merrimac, with the 
junction in the vicinity of Cambridgeport. A possible alter- 
native course for the Charles would be northeastward from 
Waltham, along the line of the Fitchburg railroad, joining the 
Merrimac near Spy pond. Such a course would avoid several 
difficulties which arise in connection with the Watertown route. 
Tertiary History of the Charles River System. 
Having outlined the geography of the region included in 
the basin of the Charles river as it existed at the beginning 
of the Pleistocene period, it is next in order to go back to the 
Tertiary period and show how, the Pleistocene system of drain- 
age could be brought about. 
When, at the close of the Cretaceous period, the recently 
formed peneplain was raised gradually out of the sea, a drain- 
age system must have been established following the normal 
seaward slope of the sediments, and entirely indifferent to 
the structure of the underlying hard rocks. It would be use- 
less to attempt to decipher the courses of these oldest streams. 
The last remnants of their valleys have long since disappeared, 
and they are as likely as not to have flowed at some points 
over the tops of our highest hills. Following the laws of devel- 
opment, the streams kept continuously at work wearing away 
the Cretaceous sediments until they became superimposed upon 
the pre-Cretaceous rocks of varying hardness, in which for a 
time they continued their work of erosion. 
Remnants of some of the valleys formed in the succeeding 
epochs can be seen in numerous gaps in the highlands at pres- 
ent unoccupied by large streams. One of these gaps probably 
formed in early Tertiary time, is the marked line of valley 
which the old Middlesex canal followed from Lowell to Boston, 
and which is the site of the early Pleistocene Merrimac. This 
+Geological History of the Nashua Valley, p. 802, 
