the last till was being spread over the upper interglacial beds. here described. Professor Fairchild places the Warren beach south of Lake Ontario at 880 feet, and his next important water level, Lake Dana, at about 700 feet, both far above the highest interglacial stratified sand or clay at Toronto.'

CONCLUSIONS

One who studies the complex set of glacial and interglacial beds of the Toronto region is strongly impressed with the length of time demanded for their production. There is no reason to suppose that the withdrawal of the Iowan ice and the drainage. of the waters which it dammed were more rapid than the similar series of events following the latest ice sheet. When the Toronto beds began to be formed the water level in the Ontario valley was probably lower than now in Lake Ontario, and some erosion had already taken place in the Don valley and at other points. There had been time for the warm climate plants to return from exile in full force and for forest trees of a most varied kind, though mainly deciduous, to grow and fall on the banks of the rivers. The unios, too, had already migrated north from the Mississippi stronger in species than they are now. All this may imply as long a time after the Iowan ice sheet withdrew as has elapsed since the last ice sheet departed, before the lowest beds of the Toronto Formation were even begun.

Then came the raising of the rocky barrier at the eastern end of the Ontario basin to sixty feet above the present level, and a halt at that level while the upper sands became browned and cemented with limonite. The climate grew cooler and and then ninety-four feet of clay and fifty-five feet of stratified sand were laid down at Scarboro', the eastern barrier rising meantime to 152 feet above the present level.

Then there was a halt in the elevation toward the northeast and at length a reversal of the motion, the northeastern end of the basin being depressed until the great Scarboro' lake was drained to a level probably much lower than that of Ontario at

Bull. Geol. Soc. Am., 1899, p. 31 and p. 56.

present, and river valleys were eroded through 150 or 250 feet of sand and clay and widened so as to have gentle slopes.

It will be observed that the damming of the interglacial waters is held to be due to epeirogenic changes and not to the presence of ice, since it is inconceivable that an ice dam should hold its place at the Thousand Islands during the ages of mild climate required for the growth of the luxuriant Don forests, largely composed of trees that now barely reach the southern edge of Canada.

It is not unfair to assume that the time after the Iowan ice retreated until the commencement of the Toronto Formation was as long as from the retreat of the Wisconsin ice to the present, a time variously estimated at from 7000 to 30,000 years. The raising of the northeastern barrier of the Scarboro' lake to a height at least 150 feet above that of Lake Ontario may also have required thousands of years, if the results of Dr. Gilbert's investigations as to the rate of tilting of the present lake basins. furnish the standard. These two stages cover only the first half of the interglacial time, and probably an equal number of thousands of years were required for the depression of the outlet below that of Lake Ontario and the cutting of wide and deep valleys through the Toronto Formation.

To arrive at the total length of the interglacial period it is not extravagant to double or even triple the number of years since the last Ice age, giving estimates of from 14,000 to 60,000 years or more. It will of course be understood that the length of time since Niagara began to cut its gorge can be estimated only vaguely and that the guess at the length of the interglacial period given here is still less certain.

How long a time the later series of bowlder clays and interstratified materials, more than 200 feet thick at Scarboro', required in their formation one can hardly even guess; but one of the glacial retreats amounted probably to more than 50 miles and may alone have demanded centuries of recession and ad

vance.

The time element in the series of events described has been

somewhat strongly insisted on in this paper, since many geologists who have worked only in regions where the Pleistocene deposits are relatively simple in structure and not of great thickness are apt to underrate the importance of interglacial periods, looking on them as short episodes of retreat and advance in the history of a single Ice age. The evidence adduced here points to completely distinct Ice ages, separated by thousands of years of mild climate. It is not improbable that the present time is merely another interglacial period.

An interesting result of the action of rivers and ice is found in the change of relief in the region since the Iowan ice departed. The valley of the Laurentian river, then probably a hundred feet or more below the present level of Lake Ontario, is now replaced by Scarboro' Heights rising 350 feet above the lake and presenting the highest cliffs on its whole shore.

A summary of the best marked stages in the Pleistocene history of the region is given below, special reference being made to climates and water levels. The latter are of course not absolute levels but only relative, since the region as a whole probably underwent important elevations and depressions during Pleistocene times.

STAGES OF TORONTO PLEISTOCENE

1. Retreat of the Iowan ice sheet.

2. Interval of erosion with water probably lower than at present.

3. Don stage, warm climate trees and Mississippi unios, water dammed by differential elevation toward the northeast to 60 feet above the present lake. 4. Scarboro' peaty clays, cold temperate climate, with trees and mosses and 70 species of extinct beetles, formed as delta by Laurentian river in interglacial Scarboro' bay.

5. Scarboro' stratified sand with some trees and freshwater shells of cold temperate climate, delta completed, lake stands 152 feet above the present. 6. Water drawn off by lowering of outlet, subaerial erosion of previous beds, and cutting of river valleys more than 150 feet deep.

7. Advance of Wisconsin ice front raising the water to about 160 feet as shown by stratified interglacial clay, retreat for 50 miles and re-advance, followed by two later retreats and advances, the water finally rising 360 feet above the present lake.

8. Final retreat of ice sheet, followed by water levels of lakes Warren and Iroquois and a brief entry of the Gulf of St. Lawrence into the Ontario basin, which, however remained fresh. A. P. COLEMAN.

PROBABLE REPRESENTATIVES OF PRE-WISCONSIN TILL IN SOUTHEASTERN MASSACHUSETTS

INTRODUCTION

In the central portion of the country, where the glacial deposits are spread out in a general northward retreating series of sheets, the tills of the various ice invasions have long been differentiated and classified chronologically with a considerable degree of certainty. In New England, however, each of the prominent advances reached nearly or quite to the southern limit of the area. The repeated passage of the ice over the region, and the consequent severe glaciation to which it has been subjected, has served to remove far more thoroughly than in the region further west the evidences of pre-Pleistocene conditions and of early Pleistocene tills. Under such conditions of glaciation, the pres ervation of remnants of the early tills would be very excep ional, and it is not strange, therefore, that deposits of these early tills have not previously been found.

While severe glaciation is the rule in New England, the action has by no means been of the same severity throughout the area. The area may be divided into three parts: (1) a northern belt characterized by severe and almost universal erosion with correspondingly little deposition; (2) a middle belt with generally moderate, though sometimes locally severe glaciation, but characterized as a whole by a marked deposition of subglacial till as attested by its drumlins; and (3) a southern belt of generally weak erosion, except in the more exposed localities, accompanied by a comparatively slight deposition of till. This southern belt, the northern limit of which in eastern Massachusetts is a few miles south of Boston, is nearly or quite destitute of drumlins, rarely shows any evidences of severe glaciation such as characterizes the northern belt, and is marked by the occurrence of numerous instances of pre-glacially decayed rock surfaces.

It was while engaged in field work on the surface geology of

this southern belt that the writer first encountered exposures of till of a type entirely unlike that ordinarily prevailing over this part of New England. In composition, in color, and in weathering, the till in question was strikingly different from the ordinary buff till of the region, and had the aspect of being much older than the latter. A further study of its character and associations was found to corroborate the differences first noted, and apparently warranted the conclusion that it should be considered as representing the deposits of an ice sheet which certainly antedated the last invasion, and probably marked the earliest of the Pleistocene advances.

The area embracing these tills is located in the eastern and central portions of the Dedham quadrangle of the United States geological survey at a distance of some twenty miles south of the city of Boston. The position of the quadrangle and of the area of the till localities is shown in Fig. 1.

It will be seen from this map that the tills are situated some fifteen to twenty miles inside of the interlobate moraine near Plymouth, and at a distance of some fifty miles north of the line of the corresponding terminal moraine. This moraine, for in origin it is a unit, is usually correlated chronologically with the Wisconsin. If this is so, and there are apparently no grounds for doubting the conclusion, it is evident that the till sheet which covers the surface of this portion of Massachusetts to an average depth of perhaps five to fifteen feet, and which is clearly contemporaneous with the moraine, is likewise of Wisconsin age.

Observations on Massachusetts glacial deposits of an age earlier than those of the last ice advance have been few in number and, with the exception of occasional instances of the burial of stratified drift deposits beneath later tills, have been confined to the vicinity of the moraines along the south coast where the conditions for differentiating the glacial deposits are more favorable than in the inland area to the north.

Before considering the evidences of older tills which the writer believes he has discovered beneath the Wisconsin till sheet, mention will be made of a number of papers presenting