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<br />5 <br />in the form of a light, dry snow. About 52 percent of the annual pre- <br />cipitation falls during April, May, June and July and only about 13 per- <br />cent is received during November, December, January and February. The <br />mean monthly precipitation during the period of record for both stations <br />is shown graphically in Figures 1 and 2. <br />B. Geolo <br />Rocks of the Precambrian to early Cretaceous age underlie most of <br />the area at great depths. These deposits are overlain by the Pierre <br />shale and generally dip westward. <br />The Pierre .shale consists of a thick sequence of fossiliferous marine <br />shale, silt, and clayey sandstone, which contains numerous calcareous <br />concretions. The upper part of the formation is transitional with the <br />overlying Foxhills sandstone. , <br />'The Foxhills sandstone is yellowish-brown calcareous marine sand- <br />stone interbedded with dark-gray to black sandy shale and some massive <br />white sandstone. The zone of contact with the underlying Pierre shale <br />consists of gray sandy shale and shaly sand. The overlying non-marine <br />Laramie formation is also transitional, and contains some lignite and <br />other non-marine beds which occur in the upper part of the Foxhills. <br />The Laramie formation consists mainly of yellow-brown and gray to <br />blue gray soft carbonaceous shale and clay interbedded with sand and <br />shaley sand. It contains some crossbedded gray to buff sandstone, which <br />is slightly to well cemented, and coal, especially in the lower part. <br />The Laramie formation forms the bedrock across much of western Weld <br />County. Much of the bedrock is covered 6y unconsolidated Tertiary and <br />Quaternary deposits. The unconsolidated deposits consist of dune sand, <br />