When springs dry up an explanation is often sought for. The present story involves a cold case but the probable culprit is not far to seek. It’s another application of the new data acquired by the Minnesota Spring Inventory (MSI).
On March 4, 2016, I was mapping springs along the eastern side of Fort Snelling State Park, near the town of Mendota. I found abundant natural alcoves along the Minnesota River bluff south of the historic Sibley House. Each alcove was associated with a spring having a discharge of about one gallon per minute. So when I later saw that the fretted bluffs of nearby Crosby Farm Regional Park in the city of St. Paul, on the Mississippi River, had a similar appearance on LIDAR coverage, I expected a like bonanza.
I went to Crosby Park on a pleasant autumn day, October 24, 2018, to map these springs. I found a meandering dirt path that conveniently followed the indentations in the cliff-line, but few springs, apart from the big spring that must have provided water to the former Crosby dairy farm. The Crosby alcoves were unexpectedly dry and this was after a lengthy period of rains. The appearance and geology of the Sibley alcoves was identical except that as noted above springs were associated with nearly every one of them (Fig. 1).
Fig. 1. Juxtaposed LIDAR coverage showing (on the right) the close association between springs (yellow circles) and fretted bluff in Ft. Snelling State Park, versus the comparative absence of springs (on the left) at Crosby Farm Regional Park. Both images are shown at the same scale with about 1.3 miles of bluff line in each case.
To confirm that these observations were not merely the result of seasonal fluctuations, both spring-lines were revisited on the same day, August 5, 2019, after a lengthy dry spell. The results were the same as before.
Spring alcoves owe their origin to spring sapping. Groundwater, emerging from the head of the alcove, undermines the cliff face, causing it to collapse, the resulting debris being swept away by the flowing water. The ravine thus eats its way back into the cliff. Alcove spacing may reflect the spacing of vertical joints in the sandstone.
The river gorge was carved by upstream migration of the Glacial River Warren waterfall at the end of the last Ice Age and the sapping process began thereafter. The spring-cut alcoves thus had thousands of years of postglacial time to gnaw their way into the bluffs, more than a hundred feet in some cases.Some alcoves along the Metro river bluffs are artificial, due to limestone quarrying. But these usually have distinctive quarry faces and squarish morphology. Moreover, the Crosby alcoves are shown on the earliest maps of sufficient scale, most notably the G. M. Hopkins Atlas of 1886. Plate 7 clearly depicts the alcoves among the “Fort Street Outlots” on Stewart Avenue, which at that time was outside St. Paul city limits, and devoid of dwellings.
All these springs emanate from the St. Peter Sandstone, an easily eroded rock that’s protected above by the Platteville Limestone, which is exposed at the heads of many of the alcoves. The springs are fed by the water-table in the sandstone and issue at the toe of the slope. But if the water table in the sandstone drops for some reason, the springs would cease to flow, leaving the alcoves as paleo-discharge features. A somber row of Platteville “headstones” in Crosby Park thus remain as mute monuments to where lively springs had once disported themselves (Fig. 2).
Fig. 2. Paleo-discharge features? One of the many dry alcoves at Crosby Farm Regional Park with Platteville Limestone outcrop visible at top of slope.
A plausible explanation for the apparent dewatering of the alcoves on the Crosby side of the river is the construction of the Fort Road sewer, which runs under Fort Road, paralleling the bluff line. This sewer was carved in the sandstone, beginning in downtown St. Paul in 1873 and extending to the southwest as the Fort Road neighborhood filled up with houses and industry (Brick 2009). Unlined bedrock sewers like this are well known to act as horizontal wells, lowering the water table in their vicinity as groundwater infiltrates the tunnel. There’s nothing similar on the Mendota side.
References
Brick, G. (2009). Subterranean Twin Cities. Minneapolis: University of Minnesota Press
RSS - Posts
You must be logged in to post a comment.