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failures to date have not been hazardous, but they will present continuing maintenance prob- lems unless arrested, and they could, perhaps, lead to some blockage of traffic lanes. Northwest of Arvada, old slumps and earth- flows are abundant around the edges of Rocky Flats, The Laramie Formation and the Pierre Shale are host rocks for most of these slides. A few of these were reactivated by the May 5-6 rains, and many new ones appeared for the first time (fig. 16). Most of these slides are along road cuts, where marginally stable slopes had been oversteepened by highway excavations. Other small slides were triggered near the bot- toms of gullies where bank scour set off slumps or earthfalls, A detailed survey probably would disclose scores of small failures of this sort. East of Rocky Flats, two small landslides partly blocked the southbound lane of Colorado Highway 121, about three-fourths mile and 1 mile south of the Broomfield interchange (fig, 17), This area is underlain by highly ex- pansive soil, if the bumps and waviness in the pavement are any indication. The southern slide had begun to fail several weeks before the May 5-6 storm when cracking and minor bulging first appeared, but the major movement was triggered by the May 5-6 rain. Movement was continuing in both slides at a diminished rate during the last week of May, FIGURE 16. - Small slump in Laramie Formation on Boulder County road 2 miles west of Jefferson County Airport. Oversteepened slope and heavy rains trig- gered failure. Crown of slide is about 20 feet above roadway. Photographed on May 8, 1973. I'? ,~ " >> FIGURE 17. - Slump-earthflow on Colorado Highway 121 about three-fourths mile south of Broomfield in- terchange. Toe of landslide partly blocked southbound lane but was promptly cut back by Highway Depart- ment. Photographed on May 8, 1973. :\-fUDFLOWS Small mudflows ran down both sides of the Dakota hogback west of Denver, especially be- tween Interstate 70 and Morrison. They were abundant also on steep slopes in the foothills of Jefferson County - myriads of them scarred the new cuts of Interstate 70. In Platte Canyon at the Denver Water Board's intake structure 3 miles above the Kassler Fil- ter Plant, a mudflow engulfed the caretaker's house, which had stood undamaged for about 60 years (Quentin Hornback, oral commun., 1973, relayed by Robert M. Lindvall). Debris-mudflows ran down several of the steep-sided buttes in the Plum Creek drainage in Douglas County (John O. Maberry, written commun., 1973). Some of these were accom- panied by rockfalls. In summary, small mudflows were common consequences of the storm in hilly or mountain- ous parts of the metropolitan area wherever sparsely vegetated hillslopes were underlain by shaly, clayey, or silty subsoils. Mudflows orig- inated chiefly in the Morrison, Benton, Pierre, Denver, and Dawson Formations and in clayey or silty colluvium. They caused considerable inconvenience and some hazard where they flowed onto highways, but, in terms of total storm effects, their damage was minor. 16