Ames Knob, North Haven, Maine

BULLETIN OF TH E GEOLOGICAL SOCIETY OF AMERICA

V o l . 14, PP. 201-206, PLS. 17-18 MAY 20, 1903

AMES KNOB, NORTH HAVEN, MAINE

BY BAILEY WILLIS

(Read before the Society December 80, 1902)

CONTENTS P a g e Introduction...... 201 General features...... 201 Topography of Fox islands ...... 201 Features of Ames knob...... 202 Rock masses and relief...... 202 Details of form...... 203 Conditions and date of submergence...... 204 Post-glacial marine deposits...... 205 Summary...... 205

I ntroduction

GENERAL FEATURES

Penobscot bay, Maine, is a triangular embayment, whose inland apex is the mouth of the Penobscot river, and whose base toward the sea is 30 miles across.* The bay opens to the Atlantic, and the nearest land to the southeast is that of South America and Africa, more than 4,000 miles distant. In the mouth of the bay are many islets and several islands among them the Fox islands,f to which this note specially refers. The features here discussed relate to wave sculpture and ice sculpture, and a brief account of the topography is accordingly appropriate.

TOPOGRAPHY OF FOX ISLANDS

There are two of the Fox islands, as distinguished from islets, North Haven and Vinal Haven, the former lying north of the latter and being separated from it by the Fox Islands thoroughfare, a narrow strait. Their extent is about 10 miles from north to south and 18 miles from

♦ See chart 104, Penobscot bay, U. S. Coast and Geodetic Survey. fSee chart 311a, Fox Island thoroughfare, U. S. Coast and Geodetic Survey.

XXIX—B u l l . G e o l . S oo. A m ., Vol. 14, 1902 (201)

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east to west. The present shores are of very youthful aspect, deeply sinuous in water line, and not conspicuously remodeled by wave action either through the development of sea cliffs or the construction of spits. There is deep water close up to steep rocky shores, and shallows are found chiefly off gently sloping lands. No wave-cut benches of rock or sandbars interrupt the advance of the waves to the shore, though here and there the surf plays about isolated rocks and skerries, which in con nection with the deep and crooked channels show how irregular is the submarine surface. The subaerial surface exhibits closely corresponding features of hollows, slopes, and hillocks, which are readily recognized as forms sculptured by brooks and rivulets and modified by glacial erosion and filling, The valleys are well adjusted to weak rocks or to shear zones in massive rocks, and the hills are residual heights maintained by harder masses. The maximum altitudes slightly exceed 200 feet on the northern part of Vinal Haven, and on North Haven one knob, known as Ames, is 150 feet in elevation ; in general, however, the higher lands are from 100 to 140 feet above sea. Lower summits, as well as higher ones, are, as a rule, bare rocks, which protrude through the glacial clay and gravel deposits mantling the slopes. This mantle is so widespread and its surface has been so little modified since the retreat of the ice, that one is impressed with the recency of that event. Neither waves nor streams appear to have accomplished much in the post-glacial in terval. Distinct evidences of marine action at considerable height above the present sealevel are therefore noteworthy as evidences probably of Glacial or pre-Glacial times, and some such are presented in the follow ing account of Ames knob.

F e a t u r e s o f A mes K nob

BOCK MASSES AND BELIEF

The geology of the Fox islands has been studied by Mr George Otis Smith, and to his account* we are indebted for the facts here made use of. Ames knob and its slopes consist of andesite, which is in general hard and compact. Even where the make-up of the rock is such that it is.properly termed a volcanic conglomerate or breccia, tne original frag ments are firmly cemented. About part of this hard mass of volcanic material occur softer beds of like origin, in which tuffaceous rocks are more common and lavas less so, while on the northern side of the knob occur sedimentary rocks, consisting chiefly of shale and limestone.

* The geology of the Fox islands, Maine ; a contribution to the study of old volcanics. Skovr- hegan, Maine, 1896.

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Figure 1.—N orthern Slope of Ames Ivxon, N orth H aven, M aine View looking northeast, showing glaciated profile of the Knob and general view of North Haven island

Figure 2.—Southeastern Si.oj’e of Ames Ivnob, N orth Haven, M aine View looking northeast, showing sea cliff and wave-cut bench 80 feet above sealevel

SLOPES OF AMES KNOB, NORTH HAVEN, MAINE

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The relation of relief to these rock masses is direct. The yielding sediments are cut down to a plain, nearly to sealevel, from the southern edge of which the compact andesite rises in a steep bluff about 140 feet high. From the summit to the east, south, and west profiles descend on gentle and roughly uniform slopes of andesitic lava, flattening as they approach the shore, and the irregularities which break their uniformity appear from a distance to be fortuitous effects occasioned by very local variations in the rocks. On nearer view, however, the details of form are found to be significant of conditions of sculpture.

DETAILS OF FORM

The steep northern slope of Ames knob has already been referred to. It is in effect a cliff, the upper portion of which is bare rock, while the lower slope, consisting largely of talus, is covered with trees. It is illus trated in plate 17, figure 1, and the rounded form of the summit, due to glaciation, is strikingly apparent. Strongly contrasting with the profile shown in this view is that on the southern side of the knob, as it appears in plate 17, figure 2. Here we see a cliff, approximately 40 feet high, rising from a sloping bench, which is broken by ridges of rock and strewn with large angular fragments. The altitude of this bench at the base of the cliff is 80 feet above sea. It extends northeastward, south, and south westward to distances varying from 150 to 200 yards from the base of the summit knob, and its outer margin is marked here and there by prominent ledges rising from 3 to 5 feet or more above the grassy slope. Their altitude is approximately 60 feet above sea, but probably varies 5 or 10 feet either way from this amount. The bench enclosed between them and the base of the highest pinnacle is a much gentler slope than that above or below, and in any profile drawn southward from the summit of Ames knob to the shore it appears as the base of a distinct reentrant angle. That it is not due to a weak layer in the other wise massive rocks is apparent from the character of the numerous out cropping ledges, which are practically uniform in hardness with the summit. Glaciation has played an important part in modifying the landscape forms of the region, and one might interpret this bench as an effect of ice sculpture, such as we find where a small glacier lingers on a mountain side after the retreat of the general ice-sheet; but such effects are produced in the shadow of a hill, and this slope is exposed to all the force of the southern and southwestern sun. There are many instances on these islands of plucked ledges—that is, ledges whose south eastern or southern face is a surface from which blocks have been torn by the advancing ice, leaving steep jointed faces exposed. The southward-

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facing cliff of Ames knob might thus be interpreted, but the wide bench extending from the eastern end around the southern and southwestern sides could not have been produced by such an action. Turning, then, to consider the conditions of marine sculpture, we find that this bench is on the side which would be exposed, almost without protection, to the full sweep of the Atlantic were the islands uniformly submerged 80 feet deeper than now. The heights of Vinal Haven would form a group of islets, which would in some measure break the force of the advancing ocean surges, but they would be too scattered and too small in area to form a protecting barrier.

CONDITIONS AND DATE OF SUBMERGENCE

Pursuing the inference suggested by the above described relations of form and direction, we may consider the land submerged to a depth of 80 feet below its present level, and regard Ames knob as a rocky islet, not unlike the Sugar loaves which are now above water at the western end of the Fox Islands thoroughfare. The islands of Vinal Haven and North Haven were then to a great extent covered by water, and the group pre sented few land surfaces approaching a square mile in extent. As the general topographic features of Penobscot bay, including the existing submarine channels, are effects of subaerial erosion, it is recognized that the coast has sunk some hundreds of feet below a former high alti tude ; and it is assumed that the submergence had progressed to the level marked by the bench on Ames knob, but it is possible that the land was for an episode relatively rising. Whatever the relative changes of altitude in reference to sealevel had been, the relation became fixed for a time, during which the waves cut the wide bench which extends from the present 60-foot contour to that at 80 feet above sea. Judged by the features developed along the western shore of Vinal Haven, which are relatively insignificant as compared with this bench, the sea stood long at that position. The writer does not know of any bench of similar width cut in equally hard rocks on this coast in post-Glacial time. Having recognized that the facts indicate a prolonged episode of sub mergence, one turns to the rock ledges for some evidence of its date. They bear no glacial striae, so far as observed, even though the under surfaces of large loose blocks which were partially protected from the weather were examined; yet the ledges resemble in subangular charac ter of profile the glaciated form on the higher knob, and the absence of striae is easily accounted for by the obvious effects of scaling and of frost and sun. These evidences of ice action upon the supposed wave-cut bench are so general that one is convinced the surface was exposed to glaciation, and it follows that the episode of marine attack preceded

FiflURK 1 .—S o u t h e r n Si,opr of Ames Knob, N orth Haven, M aine Showing in profile against the sky a wave-built bar which connects the Knob with an outlying ledge

Figure 2.—G ravel Pit in M arine Deposit of G lacial Stuff Locality is northeast of Ames knob

SLOPE OF AMES KNOB AND PIT IN GLACIAL GRAVEL

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the advance of the ice-sheet over this district. It may have been earlier than or contemporaneous with the initial stages of the latest glacial episode.

P ost-G l a c ia l M a r in e D epo sits

Were the above elements of form and sculpture unsupported by evi dence of marine sediments, the case could be presented with no stronger conclusion than a probability; but there are such deposits, and they shed additional light on the date of submergence. Two hundred yards south by east from the summit of the knob there is a prominent outlying ledge which is connected with the base of the knob by the long, sloping profile of a wave-built bar (see plate 18, figure 1). Its uniformity is in terrupted by many ledges of rock in place, but between them, and par ticularly toward its lower end, the form of a bar is conspicuous. It is composed of small pebbles, many of them thoroughly waterworn, and the material on the whole resembles that of the occasional pebbly beaches along the present coast. Gravel of similar character is to be detected here and there throughout the pasture slope, but as it could readily be explained as the wash of waters from the glacier, it would not be significant were it not in the same locality built into the unmistaka ble submarine bar. In the material constituting this bar there are striated stones clearly of glacial origin. On the northeastern side of the knob, at a point where converging currents sweeping in from the south and southwest would meet in the lee of the rock, there is a shoulder built of gravel at a maximum eleva tion of about 40 feet above the present sealevel, that is about 20 feet below the old wave-cut bench. A pit, which has been excavated in this bank (plate 18, figure 2), shows the gravels in section, locally exhibiting steep cross-stratification in association with rather heterogeneous piling of stones and sand. The stones are subangular and in some instances striated. From among these stratified layers the writer picked out some tiny fragments of shale, too small for determination and so decayed that they crumbled with the slightest pressure. These scattered gravels occurring here and there upon the slope, the wave-built spit connecting an outlying ledge with the summit, and the embankment built below the old water-level in the lee of the rock, appear to constitute a set of contemporaneous features, all indicative of submergence during or following soon after the retreat of the ice.

S um m a ry

Ames knob, a mass of uniformly hard andesite, exhibits a peculiar bench, between 60 and 80 feet above the present sealevel, along the

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southern slope, in a position exposed to the waves of the Atlantic, were the region submerged to that level. On account of this position, other conditions of sculpture being excluded by various considerations, this bench is attributed to wave action. From its extent it is argued that the duration of submergence was long. Rock ledges outcropping on the bench exhibit glaciated forms, and it is inferred, therefore, that the bench existed at the time of the ice advance. The deposits of gravel, built into a characteristic spit and embankment and containing striated stones, show that the submergence continued or was repeated after the retreat of the ice. Hence it is concluded in general terms that the Fox islands were submerged 80 feet deeper than now at the time of the latest ice advance ; that this submergence continued during the occupation of the area by the ice, and that elevation began at a date not long after the retreat of the ice, and proceeded without notable interruption until the sea reached its present level.

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