Stonewall vicinity, Colorado

James S. Aber
Emporia State University

Monument and North Lakes

Monument and North Lakes are man-made reservoirs in the foothills of the Culebra Range. Monument Lake was constructed in the 1930s as a water supply for the city of Trinidad. It has been a popular recreation site over the years. Cretaceous strata outcrop to the east of Monument Lake, and redbeds of the Sangre de Cristo Fm. (Penn.-Permian) occur to the west. The most conspicuous geologic feature is a hogback, known locally as "The Stonewall," which can be traced for many miles along the mountain front. In this vicinity, the hogback represents the Dakota Sandstone and Purgatoire Formation, as described below.

Dakota Sandstone and Purgatoire Formation (lower Cretaceous, undivided) -- In ascending order, Purgatoire Formation consists of Lytle Sandstone Member and Glencairn Shale Member. The Lytle is conglomeratic, cross-bedded sandstone, 40-60 m thick, which forms a prominent hogback. It unconformably overlies the Jurassic Morrison Formation. The Glencairn is black, fissile shale and sandstone, 8 m thick. The overlying Dakota Sandstone is 10-20 m thick, cross-bedded, brown sandstone. Total thickness of the interval is 60-80 m (Lindsey 1996).

The Dakota Sandstone and Purgatoire Formation are tilted up in a vertical position, and the hogback appears like remnants of an old castle wall atop a near-continuous ridge. Traditionally the "Dakota hogback" represents the eastern margin of the Rocky Mountain foothills. In this vicinity, numerous intrusive sills are present within the rock sequence. Sills are intrusions parallel or conformable within the surrounding sedimentary strata. Some of the sills are sufficiently numerous, thick, and resistant to erosion so as to form topographic ridges. Such sills form a hogback in the Pierre Shale immediately east of North Lake (Penn and Lindsey 1996). These sills have mafic composition (basaltic andesite). Another prominent sill is present at the village of Stonewall.

Northeast view over Monument Lake with West Spanish Peak visible in the left background. To right of lake, the Dakota hogback appears like a stone wall along the ridge top. The spillway elevation of Monument Lake is 8584 feet. Image date 3/99.
View from near North Lake toward the snow-covered Culebra Range on the western horizon. Three peaks are visible here, from right to left: Cuatro (4111 m), Maxwell, and Mariquita (4086 m). Cirques are visible on the sides of these peaks. Hells Canyon is located between Cuatro and Maxwell peaks. In the foreground, Cretaceous rocks of the foothills are present. Image date 3/99.

Hells Canyon

Hells Canyon, a tributary of the North Fork Purtagorie River, drains the eastern side of the Culebra Range between Cuatro and Maxwell peaks (see photo above). The canyon is formed entirely in thick redbeds of the Sangre de Cristo Formation. It is a classic U-shaped trough carved by a glacier that left a prominent end moraine in the lower portion of the valley. On the southeastern flank of Mt. Maxwell, old mine prospects can be seen in a fault zone. Hells Canyon is accessible via a jeep trail that leads up to the mine prospects.

View from the south side toward the head of Hells Canyon with Cuatro Peak in the center background. Notice the relatively slight dip of strata on the north canyon wall. Image date 8/04.
View from Maxwell Peak down Hells Canyon toward the east with Spanish Peaks in the distant background. Image date 8/04.
Top of Mt. Maxwell at 13,335 feet altitude, demonstrated by Jay Aber. Image date 8/04.

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GO 547/ES 747 © J.S. Aber (2005).