Guidebook for the transcontinental excursion of 1912

Preface

The attempt is made in the following pages to present a concise explanatory account of the larger physiographic provinces of the United States which are to be traversed by the Transcontinental Excursion of 1912, and a more detailed statement of the local features seen along the route of the Excursion in their relation to the provinces in which they belong. The treatment of different parts of our route is unavoidably uneven, partly from lack of material, but more from lack of time available for the work of compilation. If greater emphasis is given to physiographic than to ontographic factors, this is not due to any want of respect for the importance of the second, but simply to the much greater abundance of published material regarding the first.

A number of the speciahsts whose studies are here cited or who have directly contributed to the preparation of this guidebook will be members of the Excursion for longer or shorter periods. Many details, necessarily omitted here, will be presented orally on the ground by experts well qualified to speak upon them.

W. M. D.

Part I. The Physiographic Provinces Of The United States

The Four Belts of the Eastern United States. — The eastern part of the United States may be divided into four physiographic belts, trending northeast-southwest, i. The Atlantic coastal plain, bordering the sea and consisting for the most part of imperfectly consolidated strata dipping gently southeast, partly stripped from their original inland extension and more or less elaborately dissected. The plain has, in consequence of a modern depression, been invaded by the sea along its northern part, and thus reduced in breadth or completely submerged. 2. The older or crystalline Appalachian belt, consisting of greatly disordered crystalhne rocks, usually appearing as an uplifted peneplain surmounted by subdued monadnocks and maturely dissected by numerous valleys; this belt served as the oldland to the Atlantic coastal plain. 3. The newer Or folded Appalachian belt, consisting of a heavy series of stratified formations of varying resistance and of folded structure, which like the crystalline belt has been at least once reduced to old age since the folding, and which now, after renewed regional elevation, presents the resistant strata in long, even-crested ridges, while the weaker strata are reduced to subsequent lowlands with welladjusted drainage. 4. The Appalachian plateau, consisting of the same stratified formations as the folded belt, but here lying nearly horizontal and elaborately dissected by insequent streams. The plateau gradually descends to the northwest and passes into the drift-covered prairie plains of the upper Mississippi basin.

The four belts vary greatly along their length. Each one may now be reviewed in more detail.

The Atlantic Coastal Plain. — The inner part of the Atlantic coastal plain appears to have passed through one cycle of im

^M

Fig. I. A Branch of Chesapeake Bay, in the Atlantic Coastal Plain.

erosion, and then, after renewed uplift whereby it was extended seaward, to have reached maturity in a second cycle. Still later it lost a large part of its former extension to the northeast by submergence ; it now ends at New York harbor — except for drift-covered remnants of a cuesta visible in Long island and other islands farther eastward — so that the ocean reaches and overlaps the gently descending upland or inclined peneplain of the crystalline belt along the New England coast. Various plants, common to the coast of Nova Scotia and the middle Atlantic States, are supposed by Fernald to have reached their northeastern habitat before the coastal plain was lost by sub-

THE OLDER APPALACHIAN BELT 3

mergence along the New England coast. Southwest of New York harbor, from New Jersey to North Carolina, the coastal plain widens ; its border is here interrupted by many elaborately branching embayments that result from the slight submergence of its previously eroded valleys. Some of the long, narrow digitate bays bring tidewater all across the coastal plain to the border of the crystalline belt, as will be seen in the Potomac river (or more properly, estuary) at Washington. In New Jersey the coastal plain is longitudinally belted ; a broad cuesta of small relief incloses an inner lowland, which offers a low-grade pathway from New York city southwestward to Philadelphia. In South Carolina and Georgia the coastal plain has a maximum breadth of 120 miles and is not embayed; its seaward margin is there a low, young, featureless plain, beneath which the valleys of even the largest rivers are very slightly incised.

The Older Appalachian Belt. — The greater part of the older Appalachian belt may, as already stated, be best described as an uplifted peneplain, surmounted by subdued monadnocks, irregularly placed, singly or in groups ; the peneplain is maturely dissected ; occasional areas of weaker rocks are reduced to local peneplains of a later generation. The outer or southeastern border of the belt is determined by the retreating overlap of the coastal plain strata ; the inner or northwestern border, by the sudden down-bending of the crystalline rocks under the heavy strata of the folded belt. The breadth of the crystalline belt is greatest in New England, where in spite of the depression by which some of its outer border is submerged, it measures from 130 to 180 miles from southeast to northwest ; its inner (northwestern) part reaches altitudes of 1000 or 1600 feet, and is so greatly diversified by surviving monadnocks and incised valleys that the supposed uphfted peneplain is frequently not to be recognized. A great breadth of 170 or 180 miles is again attained in North Carolina, where the inner part of the belt is as much diversified as in New England. The difference in local aspect of these two broadest parts of the older Appalachians is largely due to the glaciation of the New England area, and is particularly apparent in the valleys: in North Carolina the drainage is typical of normally developed, mature rivers and valleys ; in New England small lakes, usually held by drift barriers, are very numerous, and waterfalls abound as the result of local superposition of streams on ledges.

Interruption of the Older Appalachians. — The middle stretch of the older Appalachians, especially between New York and Philadelphia, is narrow, low, and interrupted. It is narrow, because of an inward bend of the over apping coastal plain border, so that the breadth of the crystalline belt is reduced to 50 miles or less. Even this small breadth was once still smaller, for the overlapping coastal plain strata have been worn off from their original inland extension, stripping a formerly covered part of the crystalhne peneplain ; the stripped belt is frequently traversed by superimposed streams in narrow gorges, excellent e:^amples of which occur near Philadelphia. The middle stretch is low, seldom exceeding 600 or 800 feet even along its inner border, because its uplift was moderate. Hence the older Appalachian belt does not here constitute a formidable barrier between the coast and the interior, as is the case in New England and North Carolina. Finally, the upland of the middle stretch is interrupted, for it frequently includes infolded or down-faulted areas of weaker rocks, which are now worn down to lowlands beneath the upHfted peneplain of the inclosing crystallines.

Notable among these areas of included weaker rocks are certain limestones and shales in southeastern Pennsylvania, where the resultant lowlands are famous for their fertility ; and more important in extent are the red shales and sandstones of north-

THE FOUR PRONGS OF THE OLDER APPALACHIANS 5

westward dip, which occupy a long gently curving strip that extends from north of New York city across northern New Jersey, Pennsylvania, and Maryland into Virginia. This strip is highly significant, in that it obliquely traverses the middle stretch of the older Appalachian belt, where it is narrowest and lowest, so that the crystalline uplands are here partly replaced by lowlands for 300 miles. There are occasional tilted trap sheets contained in the red shale and sandstone series, and these survive in long, narrow, even-crested, monoclinal ridges, of which the Pahsades, across the Hudson from New York city, constitute a fine example. Other trap ridges are crossed in a local excursion from New York into Pennsylvania. But the trap ridges are local and discontinuous ; the older crystalhne belt is so largely interrupted all through its low and narrow middle stretch, that instead of forming a highland barrier, the lowlands by which it is replaced serve as gateways to the interior.

The Four Prongs of the Older Appalachians. — The path of the included red-sandstone strip in the older Appalachian belt deserves particular attention. If one follows the highlands of the crystalline rocks southwestward from their broad expanse in New England to their narrower middle stretch, they are seen to be divided at Peekskill on the Hudson into two prongs by the northeastern point of the weaker sandstone strip ; a shorter and low prong on the southeast, which may be called the Manhattan prong because the greater part of New York city, originally called Manhattan, hes upon its extremity; and a longer and higher prong on the northwest, which may be named after the city of Reading, which hes just below its end in east-central Pennsylvania. Similarly, if the broad and mountainous highlands of North Carolina are followed northeastward, they are divided into two prongs by the long southern end of the sandstone strip; a long and low prong in the southeast, extending past Philadelphia to Trenton ; and a shorter and higher prong on the northwest, which terminates near Cumberland in southcentral Pennsylvania. These four prongs, the Manhattan and the Reading on the northeast, and the Trenton and the Cumberland on the southwest, serve as convenient points of reference in various local descriptions. Between them, the lowlands of the sandstone strip occupy the whole space between the coastal plain on the exterior side and the lowlands of the folded Appalachians on the interior.

The folded Appalachian belt possesses, especially in Pennsvlvania and Virs^inia, several resistant sandstones, which now

Fig. 2. Diagram of the Allegheny Mountains.

stand up with a relief of from 500 to 2000 feet in long, narrow, even-crested ridges, to which the general name Allegheny mountains is sometimes applied ; they follow sharply zigzag patterns in consequence of the gentle pitch given to the anticlinal and synchnal axes at the time of folding. The ridges are here and there cut down in water gaps by transverse streams, which connect

THE FOLDED APPALACHIAN BELT 7

the longitudinal valley lowlands eroded along the belts of weaker strata by revived subsequent streams. Far to the southwest, in Tennessee, northwestern Georgia, and northeastern Alabama, the resistant sandstones of the folded belt are thinner and the ridges are less conspicuous, as we shall see on our return route ; they are there frequently notched, so as to form lines of detached hills, known as " comby ridges." In the northeast, the folded belt narrows and is partly replaced by the plateau belt which advances from the west in northeastern Pennsylvania. It is in the synclines of the Pennsylvania section that anthracite coal occurs : the local excursion into Pennsylvania before our departure from New York, gives a view of parts of the district where the eastward advance of the plateau replaces the folded belt. Farther to the northeast, in eastern New York, the ridgemaking strata disappear, so that the folded belt is there represented by a lowland known as the Hudson valley, in its breadth of some 30 miles from the high inner border of the older Appalachian highlands to the escarpment of the Appalachian plateau, here known as the Catskill mountains. The plateau ends there. Still farther north, the crystalline mass of the Adirondack mountains replaces the plateau ; here the lowlands of the folded Appalachian belt are in large part submerged in the basin of Lake Champlain, which drains northward to the St. Lawrence ; and the valley of the St. Lawrence, trending northeastward between the highlands of New England and the Laurentian highlands of Canada, may be regarded as a distant extension of the folded Appalachians. Through the whole extent of the folded belt a moderate regional uplift has caused the streams to incise new valleys, usually mature, beneath the lowlands of the previous cycle, which therefore now deserve to be described as low uplands. This will be apparent as we ascend the Hudson to Albany on the first day of the Excursion.

The Appalachian plateau, now in the maturity of the current cycle of erosion, after having reached a more advanced stage in a previous cycle, is a region of elaborately dissected hills and | spurs which branch into a labyrinth of insequent valleys, with maximum local rehef of looo or more feet. The plateau belt is rather sharply defined along its eastern border by the abrupt change from its nearly horizontal structure to the strongly upfolded structure of the adjoining belt. The eastward advance of the plateau border, where the folded belt narrows in northeastern Pennsylvania, and the end of the plateau belt in the Catskills have already been mentioned. The plateau and the two other Appalachian belts gradually descend southwestward and disappear under the coastal plain of the Gulf States in north-central Alabama. The westward descent of the plateau to the prairies is usually gradual ; but it is accomphshed in two dissected escarpments in central Kentucky, on account of the occurrence there of two limestones beneath scarp-making sandstones. To the northeast the plateau has a much less extension than the other two belts, because its strata gradually rise on approaching the uplifted mass of the Adirondacks — the group of subdued mountains in northeastern New York — and as the lower members of the plateau series are relatively weak, they are now worn down in a west-east subsequent valley, the Mohawk valley, into which we turn westward from the

GULF OF M E XI C 0^'^ [^

Fig. 3. The Coastal Plain of Alabama.

DRAINAGE OF THE APPALACHIAN BELTS 9

north-south Hudson valley at Albany on our first day out from Xew A'ork.

Drainage of the Appalachian Belts. — The folded Appalachians are accompanied through their whole length of over 1000 miles by a lowland, which sometimes occupies only their southeastern border, as in Mrginia and Pennsylvania ; sometimes their entire breadth, as in eastern Xew York. This long-continued lowland is often called the Great Appalachian valley. Its different parts are commonly known under dift'erent names. We follow the part known as the Hudson valley from Fishkill to Albany on our first day's run ; we cross the part known as the Valley of East Tennessee after leaving Chattanooga near the end of our return journey.

The Great Appalachian valley is not a simple valley in the sense of being followed by a single river. It is a lowland of erosion that is composed of many broadly opened, head-to-head, confluent, longitudinal valleys, most of which are drained by the subsequent branches of transverse rivers. Some of these rivers deserve special mention.

Four rivers in the middle stretch of the Appalachians, the Hudson, Delaware, Susquehanna, and Potomac, rise in the Appalachian plateau and traverse the two other Appalachian belts and the coastal plain on their southeastward course to the Atlantic. A fifth, the James, rises in the central part of the Great Appalachian valley and traverses the crystalhne belt and the coastal plain. To the northeast in Xew England and to the southwest in Xorth Carolina, the broad and high areas of the older Appalachian belt are not traversed by any river ; all Atlantic rivers head in the uplands and highlands of those two mountainous districts. In the north, the inner part of the highlands and the adjacent part of the Great Appalachian valley are drained northwestward, to the St. Lawrence ; and in the lo THE EASTERN UNITED STATES

southwest, by the Kanawha and Tennessee westward through the Appalachian plateau to the Mississippi — as we shall see at Chattanooga — and by the Coosa southward across the overlapping coastal j^lain to the Gulf of Mexico.

Settlement and Boundaries. — The embayments of the Atlantic coast line, formed by the submergence of lowlands or valleys,

Fig. 4. Canyon of the Kanawha l\.i\ cr in the Allegheny Plateau, West Virginia.

determined the location of nearly all the original colonies in the seventeenth century; and with respect to these as centers of influence the boundaries separating them were deiined. The generally north-southward trend of the coast line resulted in establishing east-west boundaries in many cases ; but the east-west trend of the coast of southern New England determined the north-south boundaries between Rhode Island and Connecticut, and between Connecticut and New York; again, the northeast-southwest trend of the south Atlantic coast produced the northwest-southeast boundaries between North and South Carolina, and between South Carolina and Georgia.

The entrance of some colonies into elongated embayments,

SETTLEMENT AND BOUNDARIES ii

trending southward, cut off the westward extension of the colony next to the east : thus Pennsylvania, first settled at Philadelphia on the estuary of the Delaware, limited New Jersey to a small area ; similarly, Virginia limited Maryland, and Maryland limited Delaware. Still more striking is the effect of the north-south trend of the Champlain-Hudson valley, whereby New York gained possession of all the western background of the New England states; hence these states are small, and for the same reason Vermont was in part colonized from the west.

It is no accident that three chief ports of our Atlantic coast, New York, Philadelphia, and Baltimore, are opposite the narrow, low, interrupted middle part of the older Appalachians. It is natural to find that the broader and more mountainous extremities of the Appalachians are of slow development; Maine in the northeast, long ago settled, with an overabundance of harbors on its ragged coast, is still a wilderness in its rugged, forested interior; and North Carolina in the southwest, settled about as early as the other colonies, has since then attracted so few newcomers to its highland country that its present population is in an exceptionally high proportion native born of native born.

All the Atlantic slope was covered with forest, rarely interrupted by treeless spaces, but not infrequently devastated by fires.' The early settlers had to clear the land for their farms. The hilly uplands of the interior were the " backwoods," and the daring spirits who penetrated them were the " backwoodsmen " of the eighteenth century, the heroes of many a story of adventure. Their successors in the nineteenth century were the ^' frontiersmen " on the treeless prairies and the plains, about whom another crop of stories grew up, but of another kind. The heavy labor of clearing the trees from the forested eastern slope finds its memorials in our forms of speech to-day. The stump of a tree formed a convenient platform for political orators in rural districts ; and now a candidate for an office who makes a series of speeches before election day is said to ''stump the state." Again, the arduous work of rolling logs demanded the aid of neighbors: '' If you'll help me roll my logs to-day, I'll help roll yours to-morrow " ; and from this '' log-rolling " has come to be the phrase to indicate the exchange of political favors, even in prairie states where no actual log-rolling was ever done.

An Ancient Coastal Plain. — With the exception of Lake Superior, the Great lakes of the St. Lawrence system — Lakes Michigan, Huron, Erie, and Ontario — are systematically related to an ancient coastal plain which borders the Lauren tian highlands of Canada on the south, in that the basins of these five lakes — and of Georgian bay on the east of Lake Huron and of Green bay on the west of Lake Michigan — he in lowlands on the inside and outside of a fairly well-defined cuesta ; the two lowlands and the intermediate cuesta being the necessary result of long-continued erosion on the heavy series of strata, dipping very gently southward and of varying resistance, of which the ancient coastal plain consists.

The Laurentian highlands seem to have served as the oldland and foundation for the coastal plain strata. The highlands consist of very ancient crystalline rocks, enormously deformed, which had been reduced to small relief before submergence caused a change from the erosion of their ancient land surface to the deposition of the overlapping marine strata. Following the later uplift of the compound mass, the stratified formations have, during an undetermined number of cycles of erosion, been broadly stripped from their foundation ; finally in the cycle of erosion preceding the glacial period, two important

THE NIAGARA CUESTA AND ADJACENT LOWLANDS 13 groups of weak strata near the base of the whole series were reduced to broad lowlands, while an intermediate formation of resistant hmestone — known as the Niagara limestone, because it forms the capstone of Niagara falls — survived as a broad cuesta of low relief.

The Niagara Cuesta and the Adjacent Lowlands. — The boundaries of the oldland, the inner (northern) lowland, the Niagara cuesta, and the outer (southern) lowland do not through their whole length trend directly east and west, but — all maintaining a rude parallehsm — turn well northward in the middle of the Great lakes region. To the west of this turn, the Laurentian oldland extends southward into the highlands of northern Wisconsin, separated from those of Canada by the unsystematic and unexplained basin of Lake Superior. Similarly, to the east of the turn, an area of crystalline rocks forms the mountainous mass of the Adirondack s in northeastern New York, separated from the Canadian highlands by the depression along which the St. Lawrence river flows northeastward to the Atlantic.

The cuesta-making Niagara limestone thins and weakens westward, and its cuesta effectively disappears in the neighborhood of the Mississippi river. Likewise, it thins and weakens eastward, and its cuesta vanishes in west-central New York.

Fig. 5. Ancient Coastal Plain of Wisconsij

Thus in the broad lowland of central New York the inner and outer lowlands become confluent, but at the same time the strata of the Appalachian plateau gain strength and thickness eastward,

and their dissected margin incloses the lowland on the south; the plateau culminates northeastward in the Catskill mountains, rimmed on the north by the Helderberg escarpment ; here a branch of the Erie-Ontario lowland, in the form of the subsequent Mohawk valley, passes between the Helderberg escarpment on the south and the slope of the Adirondacks on the north, as has already been described.

Relation of the Great Lakes to Glaciation. — It appears probable that in preglacial time — again with the exception of Lake Superior — no lakes existed in this region. If Lake Superior occupies a graben, as has been supposed, its date of origin is unknown. The basins of the other Great lakes seem to be closely associated with glaciation; but the share of crustal warping, glacial erosion, and drift obstruction in producing them has not been determined. Lake Ontario, Georgian bay, and Green bay occupy depressions in the inner lowlands; Lakes Erie, Huron, and ^Michigan, depressions in the outer lowland ; and the Niagara cuesta, often notched into discontinuous rehef, separates the two groups ; its escarpment faces the lakes in the inner lowland ; its back slope descends gently beneath the lakein the outer lowland.

EiG. 6. Ancient Coastal Plain of Ontario and New York.

LOB ATE IVIORAINES AND THE DRIFTLESS AREA 15

The Prairie Plains. — Repeated invasions of Laurentian ice sheets southward across the region of the Great lakes into the plains of the upper Mississippi and Ohio drdinage river systems, produced extensive sheets of till, often 30, 50, or more feet in thickness, by which the small preglacial relief of most of this region has been very largely concealed. The surface of the till sheets presents the aspect of a plain for scores of miles together, and the greater part of our prairie plains are of this origin. Sometimes the till surface is gently undulating, forming the so-called rolling prairie ; occasionally, it is molded in drumlins, as in central New York and in southeastern Wisconsin. 'Terminal moraines in belts a mile or more in breadth, forming irregular hills and hollows of low relief, with erratic bowlders and lakelets in abundance, mark successive pauses during the final glacial retreat, and thus separate the outer till sheets of earHer development from the inner and later sheets. The moraines are of small rehef, but they are often the only hills that are to be seen over long distances on the j)rairic i)]ains. Some of the outermost till sheets are maturely dissected even by small insequent streams, as in southern Iowa and northern Missouri. All the inner sheets still preserve their initial form, hardly affected by normal postglacial erosion, except in the immediate vicinity of the larger streams.

Lobate Moraines and the Driftless Area. — The advancing ice sheet found a greater uncvenness of surface in the region of the Great lakes than farther south; hence the ice became lobate near its margin on the prairies ; it advanced farther along the axes of depressions which favored its movement, and suffered retardation where uplands or highlands stood in its way. As a consequence of this, the terminal moraines are arranged in curves convex southward where the ice lobes i)ushed forward; they unite in northward-pointing cusps where two ice lobes i6 came together, as was first recognized by Chamberlin. Thus the Great lakes, occupying the deepest parts of the depressions along which the ice lobes advanced, and the moraines which looped around the ends of the lobes, have a strikingly sympathetic arrangement.

A singular result followed from the retardation of the ice sheet on the highlands south of Lake Superior, between the far-advancing ice lobes of Lake Michigan on the east and of Lake

Fig. 7. The Glaciated Area of the Northeastern United States.

Superior on the west. A roughly triangular space, now shared by northwest Wisconsin, northeast Iowa, and southeast Minnesota, remained unglaciated ; it is known as the Driftless area, and has been well described by Chamberhn and Sahsbury. The ice on the north could not reach it ; the ice on the east and west passed by it and closed around it on the south. In an excursion from Madison, Wis., we shall enter the eastern part of the Driftless area, traversing part of the terminal moraine of the Green bay glacial lobe on our way out and back. Within the area of this lobe are many drumlins, the axes of which diverge to the right and left from the mid line of the lobe, thus indicat-

SHORE LINES AND OUTLETS OF PROGLACIAL LAKES 17

ing that the motion of the ice was deflected towards its free margin. Another important drumhn area occurs in west-central New York, which we shall see on our second day out.

Shore Lines and Outlets of Proglacial Lakes. — It should be remembered that the divide between the St. Lawrence and the Mississippi systems is vaguely determined for much of its length by the "height of land," a broad swell of the till plains, not far south of the Great lakes ; and that from the height of land the surface slopes gently northward. During the disappearance of the Laurentian ice sheet, temporary proglacial lakes of increasing size occupied the space between the height of land and the retreating ice front. They had south-flowing outlets, most of which were tributary to the Mississippi system.

The proglacial lakes tended to increase in area as the ice melted back, but the higher lakes abandoned their southward outlets and decreased in size when the retreat of the ice margin allowed their waters to drain into or to become confluent with their lower neighbors. At the same time a slow rise of the land in the northeast — proved by the now inclined position of the shore Knes left by the proglacial lakes — caused frequent changes in lake outline and discharge. Many subordinate geographical features of to-day attest these singular events of the past, and are best described in terms of their origin.

The south-flowing outlets of the longer-lasting lakes eroded well-defined channels through the till sheets of the prairie plains, across the height of land and beyond, to different members of the Mississippi system. The little streams that now enter the channels from the prairies on either side are strikingly underfit ; not simply because the streams are much narrower than the channels, but because the small meanders of the streams cannot have produced the much larger, smooth-sided curves of the channels. The channels indeed maintain full size above the head of the present streams up to the height of land, north of which the young or submature shore hnes, formed by the lake that fed the large outflowing rivers, may be traced right and left with greater or less distinctness. Silt deposited from the lake waters sometimes cloaks the underlying till of the submerged areas and forms a soil of great fertility.

The shore lines of the proglacial lakes sometimes took the form of offshore sand or gravel reefs (bars), which now appear as low ridges, sometimes the form of low shore cliffs ; in drumlin districts, there is an alternation of cliffs and reefs. Although, Hke the terminal moraines, the shore hnes are features of small relief, they are significant in a region where the surface is so generally a smooth or gently undulating plain. We shall see some of the abandoned shore lines south of Lake Erie on the fifth day of our Excursion ; and again in North Dakota on our eleventh day.

In some cases the south-flowing outlet of a proglacial lake was abandoned in favor of a lower westw^ard or eastward outlet along the ice margin to a neighboring lower lake ; here the channel that was eroded on the disclosed land surface between the lakes ends in a delta plain that was formed in the lower lake basin. Rapids and waterfalls were not infrequently developed along the outlet channels, where resistant strata were discovered beneath the till. As the further retreat of the ice front laid bare lower and lower land between two lakes thus related, the first connecting outlet would be abandoned for a second one farther north ; and so on, as long as these conditions were maintained. The level at which the second outlet began to flow must have been a little lower than that down to which the first outlet had then been eroded ; thus a series of lower and lower outlets might be formed across a land surface between two lakes. We shall see some channels of this kind