The maturely dissected and recently glaciated uplands of New England are now somewhat depressed with respect to sea-level, so that the sea enters the valleys, forming bays and estuaries, while the interfiuve uplands and hills stand forth in headlands and islands.
The piedmont belt may be described as a maturely dissected peneplain over much of its extent; it is indeed one of the best examples of that class of forms. Its uplands are of fairly accordant altitude, which gradually decreases from 500 to 1000 ft.
Indeed, the original interior border of the plain has been well stripped from its inland overlap; the higher-standing inner part of the plain is now maturely dissected, with a relief of 200 to 500 ft., by rivers extended seaward from the older land anti by their inntimerable branches, which are often of insequent arrangement; while the seaward border, latest uplifted, is prevailingly low and smooth, with a hardly perceptible seaward slope of but a few feet in a mile; and the shallow sea deepens very gradually for many nules off shore.
The southernmost drift sheets, as in southern Iowa and northern Missouri, have lost their initially plain surface and are now maturely dissected into gracefully rolling forms; here the valleys of even the small streams are well opened and graded, and marshes and lakes are wanting: hence these sheets are of early Pleistocene origin.
The lowland is enclosed by an upland or cuesta, known as Chunnenugga Ridge, sustained by partly consolidated sandy strata; the upland, however, is not continuous, and hence should be described as a maturely dissected cuesta.
Near the Colorado river the dissected cuesta of the Grand Prairie passes southward, by a change to a more nearly horizontal structure, into the dissected Edwards plateau (to be referred to again as part of the Great Plains), which terminates in a maturely dissected fault scarp, 300 or 400 ft.
The peneplain is no longer in the cycle of erosion that witnessed its production; it appears to have suffered a regional elevation, for the riversthe upper Missouri and its branchesno longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level.
When this old-mountain lowland was broken into blocks and the blocks were tilted, their attitude, but not their structure, was monoclinal; and in this new attitude they have been so maturely re-dissected in the ne~v cycle of erosion upon which they have now entered as to have gained elaborately carved forms in which the initial form of the uplifted blocks can hardly be perceived; yet at least some of them still retain along one side the highly significant feature of a relatively simple base-line, transecting hard and soft structures alike, and thus indicating the faulted margin of a tilted block.
The Cascade Range is in essence a maturely dissected highland, composed in part of upwarped Colombian lavas, in part of older rocks, and crowned with several dissected volcanoes, of which the chief are (beginning in the north) Mts Baker (Io,827 ft.), Rainier (14,363 ft.), Adams (12,470 ft.) and Hood (11,225 ft.); the first three in \Vashington, the last in northern Oregon- These bear snowfields and glaciers; while the dissected highlands, with ridges of very irregular arrangement, are everywhere sculptured in a fashion that strongly suggests the work of numerous local Pleistocene glaciers as an important supplement to preglacial erosion.
The Sierra Nevada may be described, in a very general way, as a great mountain block, largely composed of granite and deformed metamorphosed rocks, reduced to moderate relief in an earlier (Cretaceous and Tertiary?) cycle of erosion, sub-recently elevated with a slant to the west, and in this position sub-maturely dissected.
Although this diversion of the stream may be an artificial development of a natural channel, and undoubtedly dates from a period long prior to recent Persian occupation, it appears that the later arrangements have been more maturely and better organized than those carried on by the predecessors of the amir of Kaian.
The northernmost part of the coast ranges, in Washington, is often given independent rank as the Olympic Range (Mt Olympus, 8150 ft.); it is a picturesque mountain group, bearing snowfields and glaciers, and suggestive of the dome-like uplift of a previously worn-down mass; but it is now so maturely dissected as to make the suggested origin uncertain.
A little north of the centre of the valley rise the Marysville Buttes, the remains of a maturely dissected volcano (2128 ft).