BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 29, PP. 235-244 JUNE 30, 1918 EXPLANATION OF THE ABANDONED BEACHES ABOUT THE SOUTH END OF LAKE MICHIGAN1 BY 0. FREDERICK WRIGHT (Presented in abstract before the Society December 28, 1916) CONTENTS 1 a^e I’eat deposits between the second and third beaches................................ ... 2:iT 23S Supposed changes of land levels........................................................................ Supposed earlier opening of the Sat; outlet....................................................... 240 Effects of the diversion of the water in the glacial lakes in the Ei•ie- Ontario Basin...................................................................................................... ,241 Glacial and clay deposits underneath Chicago................................................ 24:; Provisional estimates of glacial time afforded in this area........................ 244 D e s c r ip t io n o f t h e B e a c h e s Three abandoned postglacial beaches at the south end of Lake Michigan have been known for many years. In 1870 Dr. Edmund Andrews de­ scribed them in a very elaborate paper published by the Chicago Academy of Sciences. Later, Mr. Leverett, in his monograph, “Illinois glacial lobe,” and Mr. William C. Alden, in his Chicago Folio of the U. S. Geological Survey, have collected the facts in very full measure. From these and other published observations it appears that, surrounding the south end of Lake Michigan from about the vicinity of Waukegan, on the west side, and extending indefinitely northward on the east side, there is an abandoned beach approximately 60 feet above the level of the lake. This is called the Glenwood beach. Twenty feet lower, or about 40 feet above the present level of the lake, occurs what is called the Calumet beach. Twenty feet lower still, or approximately 20 feet above the level of the lake, occurs the Tolle-ston beach. These are shown on the accompanying map (figure 1), compiled 1 Manuscript received by the Secretary o£ the Society March 10, 1918. (235) 236 236 G. G. F. WRIGHT--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ABANDONED BEACHES ABOUT LAKE MICHIGAN Compiled and drawn by Miss Lonie Shedd. The broken lines south of Galewood and Rosehill moraines indicate the probable extension of those moraines supposed to have been washed down by the waves of Lake Chicago. For a description of the beaches, see text. DESCRIPTION OE THE BEACHES 237 from the Chicago Polio of the region south of the lake. All these beaches are interrupted by the Chicago outlet along the line of the present drain­ age canal, this outlet having served its purpose during the formation of them all. P e a t D e p o s it s b e t w e e n t h e S ec o n d a n d T h ir d B e a c h e s The facts most difficult of explanation connected with these beaches are the accumulations of beds of peaty material underlying the second (Calu­ met) beach. These were noticed by Doctor Andrews at various places, especially near Evanston, where they not only underlie the second beach, but “extend eastward across the interval between It and the third beach. Its level is no higher than that of the third beach, being only 12 to 15 feet above the present level of Lake Michigan. The peat is immediately overlaid by about five feet of sand, above which there is a bed of coarse gravel. The gravel is thin near the borders of the bar, but has a thickness of 10 or 12 feet at the highest part It is capped by a thin deposit of sand, and has also layers of sand interbedded in its thickest parts. The presence of this gravel makes it certain that the old marshy land surface has not been buried by the drift­ ing of material from the lower beach. There seems no escape from the con­ clusion that the lake stood at a lower stage than the level of the second beach before that beach and the bar under discussion were formed.”2 Also, according to Leverett: 8 “For a few miles in the vicinity of the State line between Indiana and Michigan there are exposures of peaty material along the blufE of Lake Mich­ igan at levels ranging from about 15 feet above the lake down to the water’s edge. Near Michigan City peaty layers Just above the water’s edge are nearly continuous for a distance of a mile or more and occur at frequent inter­ vals from Michigan City to the Michigan State line. Above the peaty beds pebbly sand in places reaches an elevation of 30 feet above the lake, or nearly to the level of the second beach. The peat appears, therefore, to have been developed prior to the formation of that beach, and probably has the same age as that noted near Evanston, Illinois.” Later, however, in Monograph L III, Mr. Leverett hesitates about ac­ cepting this evidence as conclusive. Speaking of the Evanston peat deposits, he suggests that “a bar might beextended out over a peat deposit standing at the same level as the lake and press it down and thus give it a lower level than it had while in process of growth. At the Evanston locality this interpretation would seem very plausible, for the bar was built out into water of considerable depth by southward-moving currents.” 4 2 Leverett : Illinois glacial lobe, p. 445. » Ibid., p. 445. * Ibid., p. 356. 238 G. P. WRIGHT— ABANDONED BEACHES ABOUT LAKE MICHIGAN I t is difficult to see the force of this suggestion of Mr. Leverett. How could a thickness of 10 or 13 feet of gravel, capped by a thin deposit of sand, be pushed out into water of considerable depth to cover a deposit of peat that was originally at the level of the southward moving currents which deposited the bar? The conception is impossible. We shall be justified, therefore, in accepting the original conclusions of Doctor Andrews and Mr. Leverett, which were formed when these Evanston de­ posits were all exposed. Unfortunately, at the present time the growth of the city has so modified the shore that the facts are not now open to inspection. Speaking further of the peat deposits near Michigan City, he says: “The sand evidently was deposited during the development of that beach and the peat is certainly as old as the beach. The beach may have been ex­ tended out over a peaty deposit, as was suggested in the case of the JJvanston deposits, but the conditions on the whole do not strongly favor this view.” 5 To establish the early date of the peat, Mr. Leverett would demand the discovery of valleys which entered the lake at this lower stage, at a level below the Calumet beach, and which had been built across by the Calumet beach; but in the lack of such evidence it is not necessary to give it much weight. T he Seeies of Moraines As will be seen from the map, the' outlet has two branches coming to­ gether at the Sag. These are separated by Mount Forest Island, which is in the main a moraine deposit. From the Sag westward through Lemont to Lockport the channel is on a dead level, running over a rock shelf 8 feet above Lake Michigan. At Lockport it descends through Joliet and some distance below, 35 or 40 feet in a few miles. At the foot of this descent there are immense gravel deposits (including many boulders a foot or more in diameter) on the west side of the Des Plaines River, rising 60 feet above the river plane and covering fully a square mile. The gravel is also 40 feet in depth below the river plane. Properly to interpret the history of this outlet, we must consider the series of moraines on the west side of the lake. The outer moraine is many miles in width, extending all around the south end. It is called by Leverett the Valparaiso moraine. North of the outlet there are two or three narrow parallel moraines extending from the north, but at present not reaching the south end of the lake. Numbering from the west, the Gale wood moraine is separated from the Valparaiso moraine by a valley 2 or 3 miles wide, through which the Des Plaines River flows. A little “ Ibid.. p. ?56. THE SERIES OF MORAINES 239 farther east the Rose H ill moraine is found, but does not at present pro­ ject quite so far south. The space between this and the Galewood mo­ raine is occupied by the Chicago River. Going farther north, there are remnants of a parallel moraine above Evanston. These all seem to be lateral moraines formed when the ice in shrunken quantities extended toward the south end of the lake, but the erosion of the water has very likely removed their extreme southern ends, so that their existence can only be inferred; but Mr. Lcverett thinks it not at all improbable that the Rose H ill moraine extended southward to Blue Island, which is cer­ tainly a moraine formation 6 miles long, since the rock does not appear underneath it until a depth of 50 or GO feet is reached, while it rises more than 60 feet above the lake level. It would seem also likely that the Galewood moraine extended to Mount Forest Island, which is deeply covered with moraine material. As a partial proof of this, it is to be noted that the 60-foot terrace extends southward from Galewood through Oak Park well on toward Mount Forest Island. H ist o r y o f t h e C h ic a g o O u t let It is true that this 60-foot beach through Oak Park, like the 40-foot beach which extends toward Blue Island, is composed of stratified sand and gravel; but as the erosive agencies of the lake when at its higher levels probably operated for several thousand vears, and as these agencies are known at the present time to be eating into the bank at rates varying from 2 to 3 feet a year, there was ample opportunity for them to level these narrow moraines and so in part to account for the material forming the present Glenwood and Calumet beaches in that vicinity.