THE NESSON ANTICLINE, WILLIAMS COUNTY, NORTH

DAKOTA.

By A. J. COLlier.

INTRODUCTION.

The rocks of North Dakota in general lie nearly flat; anticlines or domes are not easily detected, and the structure is generally considered so simple that in most reports it has been described in a single paragraph. For this reason the following description of a fairly well marked anticline or dome and of an artesian well yielding a small flow of gas a few miles from its crest may be of interest. This anticline, here named the Nesson anticline from a small village in Williams County, in the northwest corner of North Dakota, is about 30 miles east of Williston, 13 miles southeast of Ray, and 80 miles west of Minot and was discovered in 1917 by a United States Geological Survey party which mapped in detail the outcrops of the lignite beds in the Ray quadrangle, in cooperation with the North Dakota Geological Survey.

FIELD WORK.

The field work on which this report and the structure map Pl. XXVI) are based included a very accurate survey with plane. able and telescopic alidade of the lignite beds exposed on the orth and south sides of Missouri River in the Ray quadrangle, vhich covers the west half of the area described. The results of his work are satisfactory in all parts except T. 155 N. and the north lalf of T. 154 N., R. 96 W., where heavy deposits of glacial drift and lluvium conceal the bedrock. In the area east of the Ray quadangle the field work was not nearly so complete, and consisted f a single line of observations without instruments, made on a rip down the north side of the Missouri from Nesson to Sanish Locations and altitudes given for this area are deved from maps published by the Missouri River Commission.1 he exposures of bedrock in the eastern part of the area are good, owever, and an instrumental survey of the lignite beds north and uth of the river would probably make it possible to determine le shape of the anticline a little more closely than can be done the present time.

The assumed altitudes given on the maps of the Missouri River Commission are about 67 feet too h and must be corrected before they can be used in connection with those given on the map of the Ray drangle made by the United States Geological Survey. This correction has been raade for Plate

XVI.

TOPOGRAPHY.

The Ray quadrangle has been mapped by the United States Geological Survey with a 50-foot contour interval, but for the country to the east the only contour map available is that of the Missouri River Commission, showing a small strip of land on both Isides of the river.

The surface of the region consists of a rolling upland in which Missouri River is intrenched from 300 to 500 feet. The upland is more or less covered with glacial drift, and the lowland along the river, which is in most places less than 2 miles wide, is mainly occupied by the flood plain. At Nesson, where the lowland widens to about 5 miles, a bench 3 miles wide and 50 to 100 feet above the flood plain is covered with gravel and silt. The underlying bedrocks are well exposed only in hillsides and badlands between the upland plain and the river level.

KINDS OF ROCKS.

Underlying the area described the shale and fine-grained sand stone of the Fort Union (Eocene) and Lance (Eocene?) formations reach a depth estimated at about 1,700 feet. The sandy layers in many places show regular cross-bedding due to deposition by currents of water, or more wavy and irregular cross-bedding due to wind action, and at the time of their deposition the waters were shallow. Lignite beds, fossil leaves, petrified wood, and the shells of fresh-water mollusks are abundant in the Fort Union formation The Lance formation carries in addition to these the fossil remains of many large reptiles called dinosaurs, but it contains fewer lignite beds than the Fort Union. Rocks containing fossils of this char acter were evidently deposited in fresh water. Such rocks cover to a greater or less depth a large part of eastern Montana and western North Dakota. Beneath the Lance formation is the Pierre composed almost entirely of dark-gray shale. Its fossils are al remains of marine creatures of Cretaceous time, such as Buculites Inoceramus, and marine reptiles, and show that the sea extended to this region when the mud now forming the shale was deposited. No sandstone layers are known to be present in the upper part d the Pierre shale. To the west and southwest in Montana beds d sandstone which occur far down in the equivalent of the Pierre shale are found in the Judith River and Eagle formations at a depth of about 1,000 feet below the Lance. To the east in North Dakots

'Wilder, F. A., North Dakota Geol. Survey Second Bienn. Rept., pp. 103-104, 1903.

Stebinger, Eugene, The Sidney lignite field, Mont.: U. S. Geol. Survey Bull. 471, p. 285, 192 Beekly, A. L., The Culbertson lignite field, Mont.: Idem, p. 326.

Collier, A. J., The Bowdoin dome, Mont.: U. S. Geol. Survey Bull. 661, p. 204, 1917.