_Origin of the peneplain._ This is not a constructional surface, for the rocks are greatly tilted, folded and faulted, so that the surface consequent upon such disturbance must have been complex and mountainous. Long subaërial denudation upon a folded and faulted mass when the land stood much lower than at present produced this plateau. Evidently it could be produced by denudation only at or near baselevel, for the effect of erosion upon a mass high above baselevel is to accentuate its topographic relief, not to reduce it. We naturally ask ourselves, “At what stage in geologic history did this denudation occur?” _Date of the peneplain._ The erosion which accomplished this great work must have commenced after the formation and dislocation of the Triassic beds, for the even crest line of the trap ridges, a part of which--perhaps all--were contemporaneous with the sandstones, is a part of the dissected peneplain; but to fix the date of the completion of the peneplain, we must turn to evidence presented in New Jersey.[37] There we learn that by the close of Cretaceous times, the country was eroded nearly to baselevel, and we may therefore speak of the relative position of the land and sea, to which the land was at this time reduced, as the Cretaceous baselevel, and this land surface as the Cretaceous peneplain. _Elevation of the peneplain._ In post-Cretaceous, presumably early Tertiary[38] times, the land was elevated to nearly its present height and remained at that altitude, so far as topographic evidence shows, during Tertiary times. The proofs of this elevation are the valleys which the streams have sunk below the general level. That this was not a simple uplift, but was accompanied with tilting and warping, is clear from the following considerations. The depth to which a stream can cut its valley depends directly upon its height above baselevel. If the present surface were a peneplain uniformly elevated, the head waters and middle courses of a river would not be cut so deep in the surrounding plain as its lower course. But the reverse is true of the rivers of Connecticut. The depth of the valley increases inland, being greater in those regions where the peneplain was raised the highest. A comparison of the upper and lower valleys of the Housatonic, Naugatuck, Quinnebaug, and of the Connecticut at Middletown, where it enters the plateau, and at its mouth, will give some idea of the amount of the warping. It will not give an _exact measure_ of it for several reasons: first, the upper courses of the rivers have not yet reached the present baselevel; second, the present altitude of the uplands is the result of the post-Cretaceous uplift and warping, plus a probable later post-Tertiary uplift (to be mentioned later), besides several minor oscillations, the last of which was downward, and is recorded near the coast in the drowned condition of the rivers. As has been already said,