JOHN P. BLUEMLE North Dakota Geological Survey, University Station, Grand Forks, North Dakota 58201

Early History of in Southeast North Dakota

Extensive accumulations of collapsed lake and shore sediment are present above the Herman level adjacent to the Agassiz lake plain in southeast North Dakota. The presence of these deposits indicates that ice-dammed lakes existed on top of stagnant glacial ice while Lake Agassiz was develop- ing. Lake Agassiz did not simply expand northward from its southern end; but rather, when the superglacial lakes coalesced, the resulting Lake Agassiz flooded areas along much of the length of the Herman Beach.

INTRODUCTION The Herman Beach has long been considered to mark the earliest and uppermost level reached by glacial Lake Agassiz. Even though lake deposits at elevations slightly above Herman level have been described in association with Lake Agassiz, these have always been attributed to small, short-lived lakes. Upham (1895) Figure 1. Location map showing the Agassiz and Dakota lake plains (stippled areas), and the area (diagonally ruled) of high strandlines in southeast North Dakota. The high-level lakes drained through the spillway to glacial found evidence for pre-Herman strandlines Lake Dakota while at the Norma and Cuba strands, through Lake Traverse and to the glacial River in southeast North Dakota at elevations Warren (now the River valley) while at the Fingal and Alice strands. The outlined area is shown on Fig- about 20 ft above the Herman level. He ures 3,4, and 5. reported evidence for the narrow Lake Milnor, the deposits of which cover about commonly bouldery as a result of having associated beach deposits. Silt beds occur 15 sq mi. Leverett (1932) also recognized been washed by waves along the lake shore. immediately east of the Norma Scarps in deposits associated with Lake Agassi/;, a few The highest strandlines are named for several roadcuts at elevations between 1,380 feet above the Herman level, in Minnesota. Norma Township in eastern Barnes County and 1,390 ft in eastern Barnes County. In I recently mapped an area of abou': 1,400 where they are well developed (Fig. 3). The most exposures, these silt beds are tilted and sq mi in southeast North Dakota west of, Norma Scarps occur at elevations of 1,460 contorted. Silty till with a bouldery, above, and adjacent to, the Herman Beach and 1,390 ft (the Herman Beach is at 1,125 wave-worn surface covers areas adjacent to (Fig. 1). This area, which had net been ft in this area). They are discontinuous, the Norma Scarps in most other places. previously recognized as being pari: of, or wave-cut scarps with negligible amounts of The next-lower strandline is the Cuba even related to, Lake Agassiz, is charac- terized by lake deposits, beaches, wave- West worn till, and wave-cut scarps. The highest strandlines in the area record lake levels as 80- much as 335 ft above the Herman Beach and about 20 mi west of it. 40- DESCRIPTION OF THE HIGH STRANDLINE AREA Many small, isolated exposures of yel- lowish-gray, calcareous, laminated silt and

fine sand occur throughout the 1,400-sq- ,level mi study area. The silt and sand, interpreted r —bedding as lake sediment, is flat bedded on hilltops. On hillsides, the silt and sand beds are 40- contorted and faulted. Still farther down the slopes, in the valley bottoms, material of the same composition is unbedded. In most places, the silt and sand unit is less than 5 ft Figure 2. East-west cross section across the area of high strandlines during and after deposition of lake sediment. Stagnant ice, shown on the upper diagram, was restricted to lower areas. A thin layer of silt and clay was deposited over thick. Where silt or fine sand is absent, either the stagnant ice as well as over ice-free areas. When the stagnant ice melted, the mantle of lake sediment slumped, till or gravel is found. Till surfaces are destroying the originally flat bedding (lower diagram).

Geological Society of America Bulletin, v. 8 J, p. 811-814,5 figs., May 1974

811

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Figure 3. Geologic map of southeast North Dakota (location shown on Fig. 1). Str;indlines are shown as h :avy lines. The spillway mentioned in the text is shown by the double dashed line in western Ransom County.

10 20 30 40 50 Km _l I— —I 1— _1

EXPLANATION

Glacial sediment (till) 1 a Wave-worn tiU surface 1 b Till surface, not wave- worn •

Water-lain sediment (sand andt/or gravel) .1 a Glacial out wash tb Uncollapsed shore deposits ;t c Collapsed shore deposits :rd Windblown dunes !! e Interbedded fluvial ami lake deposits (delta areas) m

Mixed till, sand, gravel, and bedded silt 3 Collapsed materials, inc luding undifferentiated lake, shore, and glacial deposits m

Silty clay and clayey silt 4 Offshore lake sediment

Scarp, which is named for the town of Cuba 1,350 ft elevation. Sandy silt deposits with ft. In places, the sand is characterized by in eastern Barnes County. The Cuba Scarp is contorted and faulted bedding occur in the slumped and faulted bedding; elsewhere, it about 15 ft high, with its base at an elevation lower areas east of the Cuba Scarp. has graded bedding, more typical of of 1,330 ft. It is a better developed wave-cut The Fingal Scarp and Beach, the next- beaches. scarp than the Norma Scarps, and it has no lower strandline, occurs at an elevation of The next-lower, well-defined strandline is associated beach deposits. Level-bedded about 1,300 ft. It is namec for the town of the Alice Scarp, which is named for the town sandy silt occurs east of the Cuba Scarp on Fingal in southeast Barnes County. A low of Alice in western Cass County. The Alice hilltops that rise about 40 ft above the beach ridge is closely associated with the Scarp formed when the lake stood at an surrounding area. As much as 30 ft of silt is 50-mi-long Fingal Scarp in several places, elevation of 1,150 ft, about the same level as exposed in T. 136 N., R. 58 W., in north- and beach sand is exposed in several sand Uphani's Lake Milnor 50 mi to the west Ransom County between 1,320 and pits near Fingal at elevations of about: 1,280 southeast. The discontinuous Alice Scarp is

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torted. Beach deposits associated with the Alice Scarp are contorted and faulted in some places The next-lower strandline is the Herman Beach at 1,125 ft; its elevation rises north- ward. The Herman Beach corresponds to the previously mapped limit of Lake Agas- siz. In only one small area in eastern Ran- som County (Fig. 3) is the lake sediment east of the Herman Beach collapsed. Here (Tps. 135 and 136 N., R. 54 W.), several large kettles occur in sand and silt deposits at an elevation of about 1,070 ft.

EARLY HISTORY OF LAKE AGASSIZ An understanding of the high-strandline area of southeast North Dakota can help us to re-examine our traditional interpreta- tions of the early history of Lake Agassiz. The high strandlines formed at the shore of a lake (or lakes) that apparently predates the known Lake Agassiz. The size of the lake responsible for the strandlines is unknown because the eastern shores were against the glacier in the Red River Valley. The strandlines formed along a lake that existed on top of a discontinuous layer of stagnant ice, which, when it melted, caused the collapse of the overlying sediment. Widespread areas of collapsed lake deposits have not been mapped in association with Lake Agassiz prior to now, but preliminary observations indicate that such deposits may be extensive in parts of Minnesota. The highest strandlines, the Norma Scarps at elevations above 1,390 ft, are apparently the result of wave action in ponds that probably drained southward to Lake Dakota through the spillway shown on Figure 4. Modern elevations in the spillway are low enough to allow this, and alternate routes to the southeast were prob- ably still covered by glacial ice. Elevations in the spillway were probably initially the same as the level of the Norma Lake and, as the spillway was cut, the lake level dropped. When the Fingal Scarp and Beach formed, the lake drained southeastward for the first time along the east side of the Prairie Coteau and into Lake Traverse to the River Warren drainage (Fig. 1). The spillway to Lake Dakota, with elevations no lower than 1,315 ft, was abandoned at that time. The manner in which the lake sediment was deposited on top of stagnant ice is best understood by examining the area im- mediately east of the Cuba Scarp. The Figure 4. Conditions during the time the Norma Scarps formed. A glacier that filled the Red River Valley dammed small, ice-marginal lakes along its western margin. Overflow from the small lakes spilled over into Lake level-bedded lake sediment there is restricted Dakota through a spillway in Ransom County. Lakes are represented by the wave pattern. to hilltops that are generally not higher than about 40 ft, so the stagnant ice must have been less than 40 ft thick and restricted 10 to 20 ft high. Surface materials between strandlines, but they are vague and difficult mainly to lower areas (Fig. 2). Sometime the Alice and Fingal strandlines consisi: of to trace, and so they were not mapped. In after the draining of the lake that formed the till, vaguely bedded sand, gravel, and silt. All some places, bedded sand and silt that Cuba Scarp, the stagnant ice melted, caus- these materials are intermixed so that it is looks like lake sediment forms a discon- ing the flat-bedded silt that had been de- virtually impossible to predict which mate- tinuous cover over stony till, which is gen- posited on the stagnant ice to slump and rial will be present in a given location. Fea- erally less than 5 ft deep. Bedding in the slide, acquiring contorted bedding. tures that may be strandlines occur in the sand and silt between the Alice and Fingal The mixture of till, sand, gravel, and silt same interval between the Alice and Fingal strandlines is commonly faulted and con- (map unit 3 of Fig. 3), found between the

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Alice and Fingal strandlines, is material that while the Fingal strandline was formed (Fig. b( dding is undisturbed, waves at the shore was deposited on stagnant glacial ice and 5). The materials might be characterized as may have cut through the stagnant ice, or the reworked when the ice melted. The large collapsed deltaic sediment. ice may have melted befo::e the beach was amount of sand and gravel in the area was Slumped and faul ted bedding in the Fingal built. probably deposited by the Maple River, and Alice beaches indicates that stagnant ice No radiocarbon dates a::e available from which flowed southward along the margin was present beneath the beaches while they within the area of high laki strandlines, but of the glacier and into the lake that existed were being built. In places where beach a radiocarbon date of l.*!,500 ± 220 yr (1-2289) was obtained from clam shells from a fluvial terrace at an elevation of 1,060 ft just below the Flerman Beach in Ransom County, North Dakota (Moran and others, 1973). This is a minimum age for the lowering of Lake Agassiz below the Herman level in that area, and the high strandlines must be older than that. A date of 14,300 ± 1,970 yr (GSC-1369) was obtained from organic detritus in lake sediment at an elevation of 1,215 ft at the edge of Lake Agassiz in Manitoba (Klassen, 1972). If Lake Agassiz was already in existence at that early dats several hundred miles north of southeast North Dakota, it is likely that the high strandlines I mapped are a: least as old, but it is nDt yet possible to associate the date with specific southeast North Dakota strandlines. It is likely that conditions similar to those inferred for southeast Nor:h Dakota, that is, small lakes dammed along the margin of the shrinking glacier, were common elsewhere along the margin of the glacier that oc- cupied the Red River Valley. The prevail- ing, long-accepted theory is that Lake Agas- siz first flooded the southern end of its basin, expanding northward ahead of the receding glacier that formed its northern shore. This theory is probably incorrect. It is likely that many small, ice-marginal lakes, similar to those I have described, formed in Manitoba, North Dakota, and Minnesota. These small lakes gradually ex- panded and coalesced as the glacier melted, resulting in a single large lake, glacial Lake Agassiz.

ACKNOWLEDGMENTS I thank Alan M. Cvancara who critically reviewed this paper. Discussions with S tephen Moran and Lee Clayton helped me to formulate some of the ideas about the early history of Lake Agassiz.

REFERENCES CITED 1 Classen, R. W., 1972, Wisconsin events and the Assiniboine and QuAppelle valleys of Manitoba and Saskatchewan: Canadian Jour. Earth Sci., v. 9, no. 5, p. 544-560. Leverett, Frank, 1932, Quaternary geology of Minnesota and adjacent states: U.S. Geol. Survey Prof. Paper 161. 149 p. Moran, S. R., Clayton, Lee, Scott, M. W., and Brophy, J. A., 1973, Catalog of North Dakota radiocarbon dates: North Dakota Geol. Survey Misc. Ser. 53, 51 p. Upham, Warren, 1895, The glacial Lake Agassiz: U.S. Geol. Survey Mon. 25 (1896), 658 p.

MANUSCRIPT RECEIVED BY THE SOCIETY Figure 5. Conditions during the time the Fingal Beach formed. When the active glacier margin receded, an area of stagnant ice remained (the dot pattern). The lake dammed along the western margin of the glacier (wave pattern) NOVEMBER 12,1973 flooded part of this area. Fluvial material delivered to the lake, especially by the ancestral Maple River, which flowed REVISED MANUSCRIPT RECEIVED DECEMBER 17, along the ice margin in Steele County, was deposited on the stagnant ice (see Fig. 2). 1973 Printed in U S A.

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