The Heart Mountain "detachment" was caused by volcanic collapse and not free sliding. These huge blocks could hang together if immersed in volcanics, but not in air. Block separation is logical during sideward volcanic collapse, but inertia and the rate required make free sliding untenable. Lack of erosion of the fault surface requires instant burial if free sliding was the cause. However, if the detachment was part of a volcanic collapse, the fault surface was never exposed. Free-moving blocks would gouge the delicate Grove Creek pavement, but the equal loading by a glacier-like collapse of volcanics would allow this stratigraphy to remain intact. Nothing in present experience moves free blocks on so large a scale on that flat a surface, especially up and over a transgre sive ramp in a free setting. A collapsing volcanic pile propelled by its profile of repose, not by the slope under it, would allow movement. Earthquake vibration is ineffective in a free slide, but extremely effective in collapsing a weak pile. The Reef Creek structure is an imbrication; the South Fork is an unloading bulge. A long dip slope with a basin-facing monocline below it, a large young volcanic pile, seismicity, a swampy toe, and artesian pressure combined to cause failure. It may have been steady-state, incremental, or catastrophic, the latter being favored.

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