GEOLOGICAL CHANGES NOW GOING ON.

A. C. LANE, STATE GEOlogist.

If you look at the second-hand of your watch you can readily see that it is moving, but a casual glance at the hour-hand would lead you to say that was fixed and unchanging. Only by observation, repeated over a length of time, would you convince yourself that this was just as much in motion as the second-hand, only at not so rapid a rate. The same principle applies in studying many of the geological changes. We are inclined to consider the earth as fixed, and streams as relatively permanent, while in fact there are changes continually going on. The rate of change is, however, so slow that it can be measured only by repeated observation at considerable intervals of time. Moreover, we hardly realize how short is the interval during which anything like exact observations have been made. The earliest surveys that can have any claim to exactness are much less than 100 years ago, so that changes which would be only slight during that time would be forty times greater if they continue at the same rate since the time of Abraham.

I want to call your attention to a few different kinds of changes which you are likely to encounter as engineers, and which are rapid enough so that observations of a few decades apart can give us some idea of their rate. I shall not speak of the magnetic variation of the dip needle, because that has been very fully treated on various occasions.

The first case to which I would call your attention is that of the wave cutting, and the wasting of the cliffs along the lake shore. This seems to be most marked on shores facing eastward, so that the west winds and currents tend to carry the material ground up by the waves off shore, and thus prevent its clogging the action. At Port Huron there is a lively controversy regarding the amount of wastage immediately north of the town. Somewhat further north, near the line between

section 18 and section 19, town 10 north, range 17 east, is a fine case of waste of the land, which was investigated for me by Dr. Gordon, who, with the assistance of the County Surveyor, Thomas Nichols, re-surveyed the coast and found the rate of erosion since 1823 to have been 5.7 feet per annum.

A distinct impression of recency in lake encroachment is gained by observing in places plow furrows cutting straight out to and across the edge of the bluff. At one point three distinct stages of cultivation were noted. First, the oldest plow furrows at right angles to the bank extending clear out to the edge, the corresponding north-south furrow having been removed by the waves. Second, outermost furrows parallel with the cliff, and within six feet of the edge, a position it would be impossible to reach by the plow at the present time. Third and last, with the outermost furrow about ten feet from the edge. These stages are all clearly defined, and indicate a distinct encroachment of the lake between each period of cultivation. Opposite the north boundary of this estate at "E," Mr. Papst, who owns the property on the north, has placed a crib of logs which has checked shore wash here, and inaugurated a stage of land building.

The old government road from Port Huron to Port Austin follows the lake shore here originally at a distance of several rods, and shows upon the map of the Lake Survey made in 1858. Well-marked traces of it can be found to where the ruts run off the bank, and on again as shown in the map given in my report for 1901. By comparing these maps of 1858 we find that of the total waste from 1823 to 1901 of 27.25 acres (over a half-mile frontage) about nine were before 1859, and the remainder since. Probably the greater rate of erosion in recent years is due to the removal of the forest protection. We find similar changes recorded at Lexington, and further north at White Rock, where we find indications of an erosion from 3 to 6 feet a year. Sometimes these observations require some skill to interpret in detail, as there may be more or less special protection to the land, or the lake may not have stood at the same height. The lake is lower now than it was along in 1885-6, and has been so ever since 1891, and in consequence there has been

a strip added to the shore in many places, whereas previous to that time there had been very active erosion. Near Harbor Beach a strip of 50 to 100 feet has been added within the last 30 years, and yet on the west line of section 7, town 16 north, range 16 east, there is a loss from the original survey of some 50 links, and on the south line from some 20 to 30 links. This shows that previous to the recent addition of land, that is, during the fifty years from 1835 to 1885, there must have been a cutting of between 100 and 200 feet. A very similar case has been called to my attention by Mr. Sears, and I have touched upon it in the Annual Report for 1902, page 14.

The rate at which the lakes cut away at their rim is, however, affected by another factor. The whole basin of land in which the lakes lie is being tilted, and is rising to the north faster than to the south. Now it is not the easiest thing in the world to prove this matter, for it requires careful and continual gauge reading at various points around the lake, continuous, like something over a year, or, at the least, every day for more than a month, and repeated after the lapse of some decades. If, for instance, we have a gauge referred to "A" bench mark at the south end of the lake, and we find that in the year 1850 for the month of August the average water level was 2.1 feet, and the year 1900 2.2 feet, and if at another point much. further north we have another gauging station "E" referred to the bench mark "D," and the elevation of that bench mark was in 1850 3.1 feet above the lake, then in 1900 if the lake is level, and there has been no tilting, the point "D" will be 3.2 feet above the level of the lake; but if it has been rising, then the point "D" will be more than 3.2 feet above the level of the lake. Now Mr. G. K. Gilbert has found by careful comparison of the old lake survey gaugings with some that he had made five years ago expressly for the purpose, that there has been such a tilting; that this tilting is not a temporary tilting is shown by its geological effect. The streams south of the line running from Port Huron to Menominee come into the lake as dead water. They are, as we often say, drowned. North of that line they rush into the lake, and have good water powers near their mouth. In the one case, the water has been receding

from them, and in the other case, backing up into them, but we have as yet by no means as much data as we desire in this matter. If anywhere near Alpena, Cheboygan, Mackinaw, or Old Fort Drummond we could find records showing the elevation of some bench mark which we could now determine above the water level years ago, it would be well worth while to arrange to have another gauge set, and see what its relation to the water level is at present, and how much it had risen relatively to points nearer the south end of the lakes.

Another kind of geological change is the filling in of the lakes. That is very noticeable in Lake St. Clair. The St. Clair Delta was, so far as I know, first surveyed under Captain Bayfield, of the British Navy, but has been surveyed at several intervals since, up to the last and complete survey by Prof. J. B. Davis. Now while Prof. Davis has told us it is extremely difficult to draw the line between land and water down there, I think we shall be able to obtain some interesting results. There are, however, numerous small lakes which are filling up, and I have great hopes that after a few years we can take some of these which have recently been very carefully surveyed for their marl deposits, and by re-survey determine the growth of such deposits, and the rate of filling of the lake.

The streams are also meandering in their valleys, cutting here and filling there. Once in a great while, as at the Titabawassee bridge at Freeland, we are able to get some definite figures as to the rate of erosion, but usually it is quite difficult. Now my main object in writing this paper is to call your attention to such changes as I have described, and to urge upon you that whenever you run across records of accurate surveys or gaugings, etc., which if repeated might give us some indications of the rate of these changes, that you correspond with the State Survey and have the facts put on record. Sometime, perhaps in many cases now, it may be worth our while to do some re-surveying to determine the rate of change.

SOME REMARKS ON WATER LEVELS.

CHAS. M. DIXON.

The fluctuations of the water levels of the Great Lakes and their connecting waters due to the action of storms, has always been a source of much expense and trouble to vessel owners and vessel captains. The carrying capacity of vessels is usually regulated by the safe draft at the place of least depth in the channel through which they are to pass, and the result is costly to the vessel interests when the place of least depth is also so situated that the fluctuations due to storms is great. When deeply laden vessels arrive at the place of least depth at a time when there is a low stage of water, they either anchor and wait for a higher stage of water, or at times they take their chance of getting over safely, and then usually make repairs later.

It is the intention to state briefly, in this paper, the conditions which have prevailed on the Detroit River for a number of years, and to suggest some means of improvement.

At the present time, and for the past several years, the place of least depth in the channel connecting the upper lakes with Lake Erie is, and has been, at the Lime Kiln Crossing in the Detroit River. The channel at this place has been dredged through a ledge of limestone, and the bottom of the river is studded with sharp-pointed pieces of bedrock which seem to have been intended to do damage to any vessel coming in contact with them. Also at this place, the fluctuations in the water surface due to the effect of storms is very great.

As the slope in the river between the Lime Kiln Crossing and Lake Erie is, under normal conditions, only about 0.5 feet, there is nearly the same fluctuation at Lime Kiln Crossing as at the westerly end of Lake Erie. High water at this place is caused by easterly winds, and low water by westerly winds. The greatest change in the elevation of the water surface during any one storm of which there is record, was on Sept. 12, 1900, when the extreme range was about 6.0 feet, the length of time between extreme high and low waters being 9 hours.