Cross Plains Terminal Moraine
About 10 miles west of Madison, in the vicinity of the town of Cross Plains (Figs. 1, 2, 46, 47) is the terminal moraine of the late Woodfordian or Cary ice advance. At Cross Plains and for a number of miles north and south the late Wisconsinan ice sheet ground to a halt on the southwestern Wisconsin uplands, marking the east boundary of the Driftless Area with a young moraine. The moraine, part of the Johnstown Moraine, extends southward in a broad curve through Adams and Sauk counties into Dane County. It follows an irregular looping course across the Baraboo Range and is partly obscured in the Wisconsin River Valley near Sauk City. Its minutely irregular course was controlled by local topography in the deeply dissected Driftless Area in the vicinity of Cross Plains from where it trends south-southeasterly to Verona, Brooklyn, Evansville, and Janesville. Its name comes from its prominent front and abrupt reentrant angle near the town of Johnstown, east of Janesville.
The deployment of the Johnstown Moraine of the Green Bay Lobe was one of the first major glacial phenomena in the state to be worked out (Fig. 16). The moraine was described by Chamberlin (1883a, 1883b:261-298) as the terminal moraine of the Second Glacial Epoch and cited as the most important discontinuity in the Pleistocene epoch in Wisconsin. Alden (1918) in his detailed reconnaissance of southeastern Wisconsin clearly defined and described the moraine and its associated features. His paper still stands as a model today. Later workers have published information on local areas of the front, but none in the vicinity of Cross Plains.
The Cross Plains area was selected for inclusion in the Reserve in part because it contains a typical portion of the Johnstown Moraine on the uplands and a typical proglacial stream in Black Earth Creek Valley, and is close to a center of population. More importantly it is the only place known to me where the terminal moraine rests directly on well exposed, weathered dolomitic bedrock and where small marginal proglacial lakes, a marginal drainageway, and a subglacial drainageway may all be seen in a small area. Most of the length of the terminal moraine in southern and central Wisconsin fronts on broad outwash plains, in large proglacial lakes, or against older drift. There the relation of the moraine to its adjacent features is clear, but the observer must visit a large area with map in hand to appreciate it. In contrast, the various glacial features associated with the moraine in the vicinity of Cross Plains are more varied and yet as definitive as one could hope to see, all preserved in a neat little package. The area is one of increasing urbanization, and preservation of parts of the front and its associated phenomena can only be assured in the Reserve.
General Description of the Moraine
Alden (1918:212-213) described the distribution and topographic relations of the Johnstown Moraine between Verona and the Wisconsin River as follows:
The above statement is a clear description of that part of the Johnstown Moraine in the vicinity of Cross Plains. When the conditions and methods of study imposed upon Alden (1918) in his reconnaissance of all southeastern Wisconsin are taken into account, his accomplishments and insight into the Pleistocene geology of the region are nothing short of remarkable. His map, published at a scale of 4 miles to the inch, could not show all the details portrayed by the new topographic quadrangles at about 0.4 mile to the inch and 10-ft contour interval nor by aerial photographs. To this day, only details of the story need be changed. Alden (1918:209-217) clearly recognized that not all of the drift in the Johnstown Moraine was deposited during the one substage, that the thickness of the drift varied markedly from segment to segment of the moraine, and that the outermost front of the Johnstown Moraine was not everywhere synchronous nor representative of equal periods of time. Alden (1918:220-222) also demonstrated that the bulk of the pebbles and stones in the moraine were derived from rocks that crop out in the vicinity and that only 5-20% were derived from Precambrian igneous and metamorphic rocks from northern Wisconsin, Upper Michigan, or Canada. Keewenaw copper nuggets from Upper Michigan and one diamond, presumably from Canada, are among the least common constituents. Dolomite, chert, and sandstone of the local formations (Fig. 3) are most abundant.
Details of the Recommended Areas
Two areas are recommended for acquisitiona northern and southern. The northern area is the north central quarter section of sec. 13, T. 7 N., R. 7 E. (Fig. 47, 48) and the southern contains portions of secs. 4 and 9, T. 6 N., R. 8 E. (Fig. 47). Several possible waysides can be found in Fig. 47, where excellent views of the Johnstown Moraine and other features may be seen. The northern area (Figs. 47, 48) contains an excellent and typical part of the terminal moraine (Figs. 49, 50), drained proglacial lakes, a marginal drainageway (Fig. 51), a subglacial channel, and weathered dolomitic bedrock with large erratics on it (Figs. 52-55). The southern area contains a representative portion of the terminal moraine breached by a drainageway that followed a preglacial bedrock valley, kettle ponds, and an outwash apron (Fig. 56).
A portion of the Cross Plains topographic map is reproduced in Fig. 47, showing the outer edge of the Johnstown Terminal Moraine, some of the fronts established during retreat (and a position occupied briefly beyond the main front), and some of the marginal lakes and outwash as interpreted by me largely from aerial photographs. The two ponds in secs. 24 and 25 are now separated by Mineral Point Road (County Highway S) (Fig. 57). They are the remnants of a former single proglacial lake that filled the basin to about 1155 ft in elevation. The lowest pass from that basin to the west into the headwaters of the Sugar River is about 1175 ft; no evidence that the former lake ever drained through it has been found. Instead it seems to have drained northward across a bedrock ridge of the Platteville limestone (Ord., formerly Trenton of Alden 1918) at about 1155 ft into the adjacent proglacial lake at the same elevation. That lake was short lived, being held in by ice that only temporarily filled the valley 0.4 mile east of the radio tower in the extreme northeast corner of sec. 24. (The gravel pit shown in Fig. 47, in the SE1/4SW1/4 sec. 18, is actually a small quarry in the Platteville-Galena group). Water from the two lakes to the south flowed northwestward marginal to the ice from the vicinity of that pit, past an outcrop of the St. Peter sandstone on the southwest side of the valley, into another small proglacial lake in sec. 13, at an elevation of about 1090 ft. The terminal moraine lies on the northeast side of that valley, although large foreign erratic boulders may be seen to the southwest of the intermittent stream. Water from the large proglacial lake in sec. 13 briefly flowed across the bedrock spur shown in the center of Fig. 48, through the drainageway indicated (Fig. 51) leaving bare weathered dolomite of the Prairie du Chien Group exposed in ridges (Figs. 53, 54) between bifurcating distributaries as the water plunged westward from the steep face. The bare dolomite is solution etched into bizarre forms (Figs. 54, 55). Large foreign erratic boulders are scattered on the dolomite (Figs. 52, 53). That drainageway apparently was occupied only for a short time by the overflowing lake waters which soon began flowing down the Wilkie Gorge and under the ice. Water from about 2.3 miles along the front of the Cary glacier thus flowed northward along the front, from one proglacial lake to another, until finally cascading to the lowland. At first it flowed into the small drained lake basin shown in the western part of Fig. 48 and thence along the margin of the ice in Black Earth Creek Valley (Fig. 47). Shortly thereafter it cascaded down Wilkie Gorge beneath the ice.
The amount of material deposited directly by the ice in this part of the Johnstown Moraine varies markedly from point to point. Alden (1918:218) records the log of a well at the home of Mr. Voss (NW1/4 sec. 30, T. 7 N., R. 8 E.) believed to be that which is 0.2 mile east of the road junction at 1166 ft on County Highway S east of the two ponds. The well penetrated 75 ft of clay and 55 ft of sand and gravel on top of the St. Peter sandstone. However, I found dolomite just 2 ft below the surface of the gently dipping slope of the outwash apron of the Johnstown Moraine, in the extreme southeast corner of sec. 24, and dolomite crops out 0.50 mile north, in the same ridge. Thus the thickness of till in the moraine seems to be no more than 40 ft at its crest which lies on the westerly rim of a preglacial valley. The moraine is even thinner to north and south from County Highway S. The amount of fill in the basin of the proglacial lake bisected by County Highway S probably is several tens of feet although no subsurface exploration has been attempted. Erratics have been found on the west side of the basin in the vicinity of the farm house. Whether carried (here by ice rafting or by glacial ice directly is not known.
The gully crossing the drained lake basin in the southeast comer of Fig. 48 exposes 8 ft of silt on 7 ft of clean, poorly sorted sand and gravel. The base of the section was not seen. The upper silt resembles loess but contains more clay and sand and is believed to have been deposited in the former proglacial lake on top of deltaic sediments and outwash. The axis of the former valley occupied by the proglacial lake in the south part of sec. 13 lies to the east of Wilkie Gorge, about in the position of the town road that descends to the north along the east margin of Fig. 48. That axis is choked with glacial debris. Wilkie Gorge exposes the Prairie du Chien dolomite (formerly Lower Magnesian limestone of Alden 1918) up to the vicinity of the town road crossing the southern part of Fig. 48.
The end moraine to the east of the gorge is only about 20 ft thick. The moraine north of the drainageway (Fig. 48) is the same order of thickness. The upper part of the Prairie du Chien dolomite, as indicated by oolitic chert and sandstone layers in dolomite, crops out on the south side of the drainageway up to 1080 ft, and locally on the flanks of the spur at about the same elevation to the north.
Black Earth Creek Valley contains many tens of feet of glacial outwash (Dury 1964:11) whose bottom has not been reached in the vicinity of Cross Plains. The gravel pit operations 1 mile southeast of town expose at least 50 ft of coarse gravelly outwash. This outwash built up in the valley choking the mouths of tributaries downstream and forming lakes or swamps in them. Whether it is all late Woodfordian in age is not known.
The late Woodfordian or Cary ice quickly retreated slightly from its maximum position in several places north and south of Cross Plains. Only a few of the retreatal moraines are indicated in Fig. 47. One is crossed in the valley by the town road that trends north along the east margin of sec. 13 (Fig. 48). It forms a conspicuous ridge trending northwesterly on top of and west from the prominent bedrock-supported "island" in Black Earth Creek Valley, 3 miles southeast of Cross Plains. That "island" formed a distinct barrier to ice flow and to the later melt water which passed westward both on the north and on the south. Small kettles with ponds are conspicuous on its north side. Many tens of feet of lacustrine sediments are found in the south channel. Loess on top of outwash and on the moraine in Black Earth Creek Valley obviously means that part at least postdates the withdrawal of the Cary ice from its extreme position at the Johnstown Moraine (Dury 1964:13).
Glacial Lake Middleton (Alden 1918, Fig. 47) has had a long and complex history which is not clearly understood. Other shorter-lived lakes occupied parts of Black Earth Creek Valley, of which only one small one is shown in Fig. 47. The complex relationship of the Milton morainic system, thought by Alden (1918) to be a retreatal phase of the Cary, to the Johnstown Moraine will not be discussed here. Even though it affected Black Earth Creek Valley directly, its ice did not reach closer than 2 miles to the Johnstown front. Its effects on the history of the recommended sites are indirect, i.e., through loess formation or climatic modification.
The southern area recommended for inclusion in the Reserve includes the SW1/4 sec. 4 and the northcentral 1/4 sec. 9, T. 6 N., R. 8 E. Because of a bedrock ridge in the northern part of the area, the Cary ice left a prominent, but not thick, moraine with steep outer face, and small kettles in the drift behind the front (Fig. 47). Relief of the kettles is only a few feet. The steep outer face of the moraine rises 100 ft above the bedrock spurs on which it fronts, but only part is drift. A deep drainageway, cutting through the moraine in the center of sec. 5 exposes St. Peter sandstone and Prairie du Chien dolomite below the moraine, as does the marginal drainageway extending southward from it. Bedrock is also exposed in the road-cuts and in the gravel pits in the outwash area shown in Fig. 47. Drilling between the gravel pits disclosed bedrock at a depth of a few feet. Thus the moraine and outwash are only a relatively thin veneer mantling a stream-dissected bedrock topography. The top of the moraine rises over bedrock highs and drops in the preglacial valleys.
Dating of the outwash has not been done. Alden (1918, Pls. 1,111) shows his boundary of older drift encompassing the outwash area and terminating against the Johnstown Moraine at about the center of sec. 5.
One of the deeper preglacial bedrock valleys may be traced from the gap in the moraine, south of County Highway PD, northeastward to Five Points and Morse Pond, and thence northeasterly to the marked steep-walled valley northwest of the WISC radio tower. Dolomite and sandstone of the St. Peter and Platteville-Galena Formations crop out on both sides of the valley which is occupied partly by kettle lakes, the largest being Morse Pond. Typically along the Johnstown front, areas of outwash are localized by preglacial topographic lows in the bedrock. There the glacier could maintain its thickness with a lower surface elevation, which in turn concentrated surface runoff at those points. The highest bedrock ridges have negligible outwash, as is seen between the two recommended areas or between the southern area and Verona where another preglacial valley is found. The one at Verona is larger and topographically lower than the one in the southern area (Fig. 47) and consequently contains far more outwash. The same principle applies to the Rock River Valley at Janesville where still lower topography concentrated many times more runoff and several hundred feet of outwash, not all of which can be attributed to the Cary ice.
The contrast of drift-mantled surfaces behind the Johnstown front with only a thin loess mantle beyond the front can be seen easily from a car. The topographic map (Fig. 47) shows somewhat more irregularity of topography in front of the moraine than behind, but the effect of glaciation close to the front has been one mainly of filling in the lower areas rather than of eroding the higher areas. Drift on many of the uplands behind the front is only a few feet thick, but in valleys it is tens of feet thick. The main valleys in front of the moraine have also been filled with tens of feet of outwash and loess-derived colluvium. Hence, only valley sides and tops of hills beyond the front show the paucity of cover. Erosion by frost action, gravity movements, and surface runoff apparently was far greater during glacial times than now, and temporary features like pinnacles and castelated spires of easily eroded sandstone are fairly commonplace within the Driftless Area. Small sandstone pinnacles may be seen 1 mile west of Pine Bluff, on the south tip of a spur of St. Peter sandstone. A larger pinnacle is in a roadside park on Highway 92, 1.5 miles northwest of Mount Vernon. Devils Chimney 2.3 miles southeast of Mount Vernon (New Glarus quadrangle) is still larger (Fig. 58). Both of these pinnacles are thought to be related to a former glacial lake that occupied the West Branch of Mount Vernon Creek, but the details of these and other pinnacles in the area are beyond the scope of this discussion. It is hoped that some may ultimately find their place in the Reserve.
The problem of whether the Driftless Area was ever glaciated is not of immediate or direct concern to the two recommended areas. They provide no evidence either for or against the concept. The solution effects in the Prairie du Chien dolomite could have been accomplished in the 13,000 years the rock presumably has been exposed, but it has not been proved that a longer time was not utilized. I suspect that the solution phenomena have been exhumed by the glacial melt water and were formed during an earlier weathering cycle. The thin young loess cover and lack of residual materials on the bedrock outside the front prove that any older loess and residuum have been removed or were never deposited. The timing of this event cannot be dated except indirectly in other areas. No evidence of older drift outside the Johnstown Moraine between the town line north of Verona and the Wisconsin River has been recognized, yet no old accumulation of residuum nor old loess has been recognized either. Why? Their removal from the hills and completely out of the drainage network as well is not explicable by normal runoff such as is experienced today. This problem will be discussed further in Chapter 11.
Last Updated: 1-Apr-2005