The Most Casual Observer Crossing Lehigh County from North to South
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PHYSIOGRAPHY By BEKJAMIN L. I~ILLER The most casual observer crossing Lehigh County from north to south note’s differe.nces in the topographic features, whereas a person traversin,g the county in a west-east direction notes few changes. It is the function of the geologist (geomorphologist) to differentiate, classify, name and explain these resemblances and differences. Numer- ous investigators have studied these forms in Lehigh C.ounty or in other parts ‘of the Appalachians. Because of the wide extent ‘of each physiographic type represented in the region, studies made in some- what .distant sections are pertinent to this discussion. There is general agreement regarding the m,ain features but not in the minor divisions and in the nomenclature. The classification of Fenneman,* adopted by the U. S. Geological Survey, is mainly fol- lowed in &is report. According to this usage, Lehigh Crounty constitutes a small portion -_ of the Appalachian Highlands which extends from Canada to central Alab’ama and from the Coastal Plain on the east to the Interior Plains on the west. This is divided into provinces and these in turn into secti,ons. Lehigh County contains portions of the following divisions : ‘W/AN PC A TEA PROW/NC,5 PKDNONT PEW Figure 4. Physiographie subdivisions of Pennsylvania. * Fenneman, N. M., Annals Assoc. of Amer. Geographers, vol. 18, pp. 262-353, 192s. 106 PHYSIOGRAPBY 107 Physiograpbic divisioas of the Appalachiun Highlands represented in. Lehigh County Province Bection Distribution Piedmont Triassic Lowlands ?oorly represented in non-char- Lowlands bcteristic form in southeastern Jortion ‘of Lower Milford and >pper S,aucon Townships. New England Reading Prong steep, hills or ridges of crystal- Uplands (Also called Appalachian line rocks, irregular in form, (Also called Mountains). separated by flat-bottimed val- Older Appa- leys of limestone. Geographi- lachimas). :ally these are South Mountains 3r the Durham and Reading Kills. This area lies south of Allentown, Emmaus, Macungie znd Alburtis. Ridge an d Porti,on between South Areas of limestones and slates, Valley Mountains ( D u r b a m Province* and Reading Hills) and (Al50 called Eittatinny (Blue) Mouu- Newer Appa- tain is called Appala- lachialzs and chian Valley an’d Lehigh FolaeaAppa- Valley. Zachians) . Kittatinny (Blue) Ridge ‘of Silurian (Shawangunk) Mountain. sandstones and conglomerates. * Some writers have limited the Ridge and Valley division to that portion of the Appalachian Highlands lying between the Allegheny Mountains (or Plateau) and the Appalachian Valley. According to this usage, Kittatinny (Blue) Mountam 1s the only part of the Ridge and Valley province present in Lehigh County. Topographic Features The main ,topographic features of L’ehigh C.ounty are so distinct that they were noted by the early lexplorers and travelers, some of whom offered fantastic explanations as to their origin. In recent years trained investigators have studied them in detail and have dis’agreed in so many points that the writer enters this field of contr,overay with extreme reluctance. He theref,ore presents descriptions of the phe- nomena and the theories of different investigators. Althou’gh prefer- ence for particular views may be shown, an attempt is made to leave the acceptance or rejection of various ideas to the individual reader and to future students. EROSION SURFACES When that stimulating and earnest student of land forms, William Morris Davis, wrote his articles on the streams of Pennsylvania and New Jersey in volumes I and II of th’e National Geographic Maga- zine he initiated the scientific investigati’on of the topographic features of this regi’on. Before Davis wrote, there were occasional notes by other geologists. In 1883 J. P. Lesley * called attention to three ._ * Pennsylvania 26 Geol. Survey Rept. D 3, vol. 1, 1883. I 108 LEHIGH COUNTY prominent plains (planes) in the region. Of Kittatinny (Blue) Mountain he said, “Its crest a nearly horizontal line everywhere about the same height (1,500 to 1,600 feet above the sea).” (p. 23.) In the following quotations he describes the slate and limestone belts of Northampton County. His descriptions are equally applicable to Lehigh County: “The slate belt of Northampton County stretches from the Delaware to the Lehigh for seven miles south of the moun- tain. It is (a) region of low flat-topped hills, trenched by a multitude of small valleys, and when looked down upon from the mountain, ap- pears like a great plain, which it really is.” (p. 27.) “The limestorze plain of Northampton County is about seven miles wide, and ele- vated * ’ ’ about 400 feet above tide ; its hill tops sometimes reach 450 feet. This plain is intersected in every direction by gently sloping vales. ’ ’ (P. 36.) A mere casual examination of the topographic map reveals four sharply distinct topographic regions. A fifth one is poorly repre- sented. From north to south these are Kittatinny Mountain, the slate region, the limestone valley, and the hills of the southern part of the county. These will be described in turn. The first three belong to the Ridge and Valley Province and the fourth to the Reading Prong of the New England Uplands. The fifth is the Triassic Lowlands. In some regions the topographic features have little relation to the stratigraphy. In Lehigh County, however, each of the erosion surf,aces (‘peneplanes ?) is almost completely confined to a particular formation or group of formations. This has resulted in a discussion as to whether the successive more or less level surfaces should be designated as pene- planes or not. At this point they will be described as topographic features only and called erosion surfaces. Their origin will be dis- cussed on later pages. Kittatinny (Blue) iVou&ain.-This even-crested mountain ridge, visible on a clear day for long distances, is the most regular and mo’st impres’sive topographic feature ,of the entire region. Viewed from the distance, it has the appearance of the edge ‘or the escarpment of an elevated plateau. When ‘one climbs it and finds that the moun- tain is a narrow ridge with the top scarcely wider than one-fourth mile any place ,and generally much narrower, the mountain then ap- pears as a barrier ridge. The hard Tuscarora conglomerates and sandstones f’orming the mountain dip to t’he n.orthwest at angles vary- in&g from 30’” to 60”. The Martinsburg shales beneath dip in the same direction, likewise at various angles. The contact between the two formations, seldom observable on account of the hillside talus, is generally about half way up the sout’hern sl.ope. The crowding of the contours sh.own on the map indicates the steep- ness of the slopes. They can be climbed by the pedestrian only with considerable difficulty. The two roads that cross the mountain ascend by diagonal and zigzag courses and finally cross the crest in notches ‘or min’or .gaps. No stream crosse,s the ridge west of Lehigh Gap within the county, so that it forms a prominent divide between the tributaries of the Lehigh R,iver and between Lehigh and Carb,on Counties. The mountain is a single ridge with a remarkably straight trend to the PHYSIOGRAPHY 109 west-southwest. Although the gener,al appearance of the mountain on the skyline is that of a straight line, closer inspect.ion and the examina- tion of the map shows considerable variation in elevations. Ignoring the gaps or notche,s, the crest varies from 1,400 to 1,601Ofeet above sea level. Some persons have been inclined ‘to suggest two levels of different altitude but since there is no marked sep.aration between the two, it seems preferable to regard these varying elevations as due either to crustal warping or to varyin, v amounts of erosional degradation. Kittatinny (Blue) Mountain may be said to typify what is regarded as a somewhat lowered peneplane remnant, the remaining portion of an extensive plane of erosion that at one time extended throughout the entire Appalachian region. It was once believed that during Cretaceous time the tops of the folded Appalachians were worn down to base level. More recently the date for this leveling has been placed in middle or late Tertiary time. Subsequent elevation has permitted the removal of the softer, less resistant rocks on both ,sides. Other similar ridges lie to the n,orthwest ‘of Kittatin.ny Mountain in adjoining counties. In some places there are several parallel ridges separated by steep-sided valleys. In every case the ridges are com- posed of hard resistant rocks and the valleys cut in shales or in a few places in limestones. Along the Susquehanna River the continuation ‘of Kittatinny Nlount.ain is called First Mountain and is followed northwestward by Second and Third mountains. Slute (Shale) Region.-The ordinary traveler in the region can scarcely fail to note comparatively flat-topped stream divides in the northern part of the county where the rocks are mainly the Martins- burg shales and slates but with some associated sandstones. In the southern portion of the band these fairly level hilltops have a general elevation of 650 to 7010feet, but going north toward Kittatinny (Blue) Mountain similar divides rise above 800 feet. Where the sandstones are well-developed, the crests rise well above 900 feet. A knob near the western end of Shochary Ridge is 1,060 feet above sea level and the apex of the ridge south of Slatington is 1,000 feet. In Lehigh County the accordance of these fairly level hilltops is less noticeable than in adjoining regions. Nevertheless the entire region would pre- sent a gently rolling surface gradually rising toward the north, if the steep-sided narrow stream valleys could be filled, with the sand- stone ridges rising above the level surface as monadnocks.