22 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



which represent the successive geologic events and to the evidence 

 upon which each is based. 



STRUCTURE SECTIONS OF SUCCESSIVE GEOLOGIC 



PERIODS. 



The Present Geologic Time, Figure 2. The section shows 

 the relatively slight relief of the valley ridges and the Highlands 

 above the Central Lowland, as compared to the former reliefs 

 implied by the eroded structures. A new cycle of erosion has 

 begun but has not yet made much progress toward completion, 

 as shown by the narrowness of the alluvial flood plains, the hilly 

 character of the Lowland on a small scale, and the steep slopes of 

 the valley walls. Uplift has therefore been geologically recent, 

 but has been of a broad and uniform nature, since the next older 

 base-level of erosion represented by the peneplain of the Central 

 Lowland is still approximately level though slighly higher in the 

 north. Its elevation on the line of the structure section is about 

 200 feet and this marks the amount of uplift. The present cycle 

 of erosion, however, although but slightly advanced, has been 

 in progress since at least the middle of the Quaternary period, 

 as the river valleys are mantled with Glacial till and floored with 

 outwash gravels, showing that they were eroded before the last 

 invasion of Glacial ice. The progress of the cycle toward comple- 

 tion is, therefore, a measure of the relative length of a part or all 

 of the interglacial stages of the Quaternary period, rather than a 

 product of post-Quaternary time. But the uplift has been so small 

 and the erosion of the rock last raised above sea level is so little 

 advanced that the results cannot be given expression upon the 

 structure section. The importance of noting its occurrence lies 

 in pointing out the relative insignificance of recent erosion, and 

 in emphasizing the fact that all the features shown in the drawing 

 are the impress of earlier geological periods, not of that in which 

 we live. 



Connecticut during the Glacial Period, Figure j. The con- 

 tinental ice sheet reached as far south as Long Island, and buried 

 all the hills of Connecticut, as well as the Catskill, Green, and 

 White Mountains. From various lines of evidence, its thickness 

 over the Central Lowland on this section line, when at its maxi- 

 mum, may be estimated as approximately a half-mile. It was an 



