Ann Arbor Of Old

Written for The Register. Let us visit the gravel bank at the foot of Thayer-st, near the Michigan Central depot. We fiod that, in the face of the excavation, which fronts to the north, but bends around on either side so as to expose faces fronting west and east, the lower levéis are exposed through a depth of ten or twelve feet, consisting of a fine, uniform gravel, its pebbles averaging about the size of peas and beans. This gravel is arranged in beds such as are deposited by the waves along the shores of our great lakes, but bed is piled upon bed as the waves along the lake shore could never pileitunless, indeed, thelevel of the lake were rising step after step through the whole depth of the gravel beds from the lowest to the highest, and in that case it would fail to duplícate this gravel bank for want of material. As a matter of fact, a rising lake would shift its gravel bank shoreward as its waters rose, and leave but one bed of moderate tbiekneS3 to ma'k its final level. Then we notice that each of these gravel beds is obliquely laminated a9 if composed of thin layers leaning up oce against the other from the shoreward limit to the lakeward. Such laminie in aforming gravel bank we know always dip toward the deeper water it is the slope of its edge under water that marks the pitch of these láminos. In all of these gravel beds, however, the laminas dip toward the south, that is, toward the hill on which the city of Ann Arbor is builc, while they rise toward the north where the valley of the Huron river is now excavated, to a depth of fifty or sixty feet beneath this level, as if the shore had been there when they were deposited. THE STRANGEST FEATURE of this bank, however, is a layer of bowlders five or six feet thick, which lies upon these gravel beds ; these bowlders, range from pebbles the size of a hen's egg to blocks that would weigh a quai ter of a ton. The general appearance of the stratam is as if it were composed of bowlders a hule iarger than a man's head. What does this mean ? These bowlders can not have come far in this shape. They aro spread like the gravel beds on which they rest, as if by the action of the same water that deposited the gravel. Yet we know that a stream of water rapid enough to have brought the bowlders here must have carried away the gravel iastsad of depositing them upon it. One possible explanation suggests itself. This bank may have been at the border of a lake with the precipitous FACE OF A CLAY HILL, containing bowlders, towering over it where the riyer vailey is now. Such a hill as that which now. rises just across the river to the north-east ; of such matenals as the bowlder clay that, cropping up through the gravel, is now beine; excavated for a road way just west of the Michigan Central depot. Such a hill as this coming down as a land slide, into the border of a lake would have its clay and finer materiaU carried away by the waves while its bowlders would remain in a stratum like this, resting upon beds that had previously been laid down. The gravel beds we know extend under pretty nearly the entire city; they composed the Cup and Saucer hill, now almost leveled on East Catherine-st. We have seen them excavated at the south end of Statest., and every excavation for the foundation of a building throughout the city exposes them, but we have not seen a bowlder bed like this elsewhere. There is another gravel pit on the west side of the city, west of the Toledo and Ann Arbor railroad track, near the foot of the hill that overlooks the city from the north-west, and beyond the valley down which flows a brook that separates the western suburb from the main part of the city. Let us visit that. Here the principal face exposed, fronts the south-east, with its south wing bending round to face the north. The total depth ot face exposed is about thirty-five feet, THE DEEPER STRATA exposed being at the south end of the pit. These deeper strata consist of sand, thin bedded, the layers succeeding each other at intervals of less than half an inch. This sand is sprinkled with scattering pebbles, ranging from the size of a butternut to that of the sand itself. The e beds have a very perceptible dip toward the south and have the appearance of beds deposited ON THE SLOPE OF A LAKE BOTTOM below the action of ordinary waves, down which a few gravel pebbles have slid frorn higher levéis. Some of these lower beds have been cut away on the south side and refilled with similar beds, having the same general slope as before. The upper bede of the exposure at the south end of this gravel pit, wBich lies wholly below an excavation made some years ago, consist of sand with a less admixture of gravel, capped wifh an unstratified talus of mingled gravel and clay from the older excavation. The northern part of this gravel pit is beneath a part of the hill not previously excavated. It faces the south-east. The lower part of the excavated face corresponding with the beds just described, is covered with loose materials that have fallen from above. Beneath this heap at the south, those beds Cnnctuded mi 2nd Page. AS-V ARBOR OF l.l. .Continue! front ist page. can be geen entering with a slope rising at an angle of ten or twelve degrees toward the north. Above this talus is ezposed a series of thicker beds of gravel similar to that exposed in the Thayerst. pit. These beds are nearly horizontal and show oblique laminationa dipping toward the south as before, but at this point a broad valley opens to the southward. Above these beds of gravel is a bed about ten feet thick of horizontal stratafied material, composed of mingled clay, sand and gravel. This has the appearance of having been spread by the action of the water upon a large maes of the clayey material which HAD FALLEN FEOM THE HILL, behind. If this is the case there should be a bowlder bed overlying the gravel at some point between this and the foot of the clay hill. Two large bowlders lie in the talus but whether deposited by nature as part of the material of the bank, or since transported on the surface and allowed to fall there does not appear. No stone larger than a Len's egg is seen in any part of these undisturbed strata. The gravely pebbles are usually of a sub-angular character, having the appearance ot fragmenta with the angles somewhat rounded ofif by friction, but usually lees rounded than the pebbles of a lake shore bed. Some of the pebbles, however, are thoroughly rounded. In a handful of this gravel taken at random we find fragmenta of slaty f hale, feldspar, mingled with mica, quartz, limestore, felsite, greenstone, limonite in the form of a sniall rounded pebble, eagily scratched away with the finger nail, thus showing its compoBition of yellow hydrated oxyd of iron, and the largeet piece in the handhil is a sort of hardened silt so soft that ït is easily broken and crushed between the thumb and finger. These pieces seem to be accidental fragmenta rather than pebbles that have escaped destruction on account of their hardness resisting a procesa of attrition. This fact may have some important meaning. Let us remember it. About sixty rods to the north-west of this gravel bank is the foot oí a hill which riges nigher than any other in the vicinity of .A nn Arbor. We will ascend it The Toledo and Ann Arbor railway cutí into its foot and here it consiste of CLAT MISOLKD WITH BOWLDERS and gravel similar to that of the gravel bed. The same material crops out by the roadside all the way to the summit and the same clay is turned up by the plow on a farm just over the crest of the hill. This is evidently older than the gravel beds and such a material ig a source from which they could have been derived. The view over the city from this point is very fine. We cannot help noticing the beautifully rounded outline and gentle slope of the hill, on which the principal portion of the city is built. As it lies there crowned with the dome of University hall and the clustered towers of the other University buildings, together with the spires of half a dozen churches and the roofs of the city, all thrust through the foliage of a foreet of thmde trees, neither nature nor art could shape thegroundto eicel i ts beauty. To the north-east and eait rise other hills similar in character to the one on which we stand. To the sou th west, and reeed - inL as we turn to the south rise others, while a way to the e ast stretches a broad valley down which the Toledo and Ann Arbor railroad is built. We remember that in that direction there is not a hill in til the distance between Ann Arbor and Toledo. AND NOW LIGHT DAWNS OH US. The Ann Arbor gravéis begin to bear a new meaning and to teil us a more definite story. There are no Iittle lakes on that level tract between Ann Arbor and Lake Erie. That has all been worked over by the waters of a greater lake and its inequalities have been wiped out Yes, it grows plain. The Ann Arbor gravéis are a river delta. The whole contour of the hill on which the city is built is just what a bank formed under water would assume. The broad valley opening to the south-east was then a continuation of Lake Erie. Here was its terminal bay, and here the Huron river formed A TDRBID TOBRENT LOADED WITH GRAVEL and mud from the loose and naked huls to the north-westward into its waters. Dropping its gravel at the river mouth it soon formed a shallow bar, as a bar is always formed at the mouth of a stream that is loaded with sediment, and the current of the river spreading over this rolled its gravel along and deposited it on the lakeward slope. Every freshet deposited there a new gravel bod and every subsidence allowed a thin coating of finer mud to settle over the bed last formed and so distinguish it from the one which was to follow. While the gravel was thus dropped as soon as the river current met the waters of the lake, the Qner sand which it bore was carried farther and dropped in deeper water, forming a f lope like that exposed in the south end of the western gravel pit which we have examined, and the clay held suspended in the waters of the great lake was carried away to be slowly deposited in its depths. No feature which this state of things should auce is lacking. JNear the northern ghore of this western terminal bay oí Lake Erie was an island, or perhaps for a time a peDÍDsula, the northwestern point of which ig-now THE OBSERVATORY HILL. If we ascend that hill we observe that the moment we me above the general level of the city we leave the gravel beds behind ug and the clay appearg at the sides of the road way. To the north-west of the observatory between that buildiDg and the river bluff, is a small tract of land, the surface of which remamg in the form that nature left it, and here the ground asaumes a short rolling irregularity of contour characteristicof a clay country that has been carved by free drainage. The whole región speaks as ucmistakably of land above the water level as of the presence ot the lake on the site of Ann Arbor, and the deposition of the gravel beds in its waters. At length, the obstruction which had retained the waters of Lake Brie at this great height, which may have conaUted of just such clay masses as the hillg from whieh these gravel and bowlder beds were washed out, broke away and the waters retired to some lower level. The Huron river was now under the necessity of FINDING A NEW CBAHNÏL, but the open bay into which it hadflowed was now blocked up with its delta of gravel. It found an outlet along the edge ot that delta, by the northern shore, to the nortbward of the island against which its gravel beds were piled. If we observe a bar at the mouth of a stream where it is now forming, we sball ueually find the water a little deeper at its extremities where it abuts against the shore than over its central parts. This seems to have been the situation here and for thig reason the river took its cour6e around the northern edge of its delta rather than over it. Be tween the old mouth of the river and the new was now a considerable difference of level, and the stream flowed over the loose and soft earth of the drift through the whole distance. Tinder these conditions t could not dootherwite thaa very quickly cut a gorge along its new channel. From this gorge ravines crept back, aad breaking down at their sides, the country in a few ycars could assume a contour ditfering but little from the picturesque hills that now exist along the valley of th Huron. At this point of its history the forest sprang up and clothed the plabs and the hillsides and a new and peaceful geological epocb was inaugurated.