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The "Michigan Stray" sand, from which most of Michigan's gas is produced, consists of a series of sand bars formed on off-shore shoals in a shallow Mississippian sea. The shoals that caught the bars are on long, anticlinal trends, the sea-floor topography being determined partly by structure and partly by erosion during a previous period of emergence. In some fields enough data are available to show the size and shape, and configuration of the bars, and in at least one the sea-floor topography has been worked out, and the cause and manner of deposition of the bar are plain. The main bar formed against a small sub-sea hill, the top of which may have protruded as an island, and a smaller bar formed on a lower shoal on the opposite side of a cross channel through which enough current pas ed to keep the channel almost, but not quite, free of sand.

The sand bodies are of some magnitude. The largest so far explored is about 8 miles long and 3 wide and held about 50 billion cubic feet of gas. Here three parallel bars were formed and eventually coalesced into a single great bar, featured by three main undulating ridges with intervening hollows. The upper surface of the sand body is strikingly similar to the topography of a present-day sand-bar area.

None of the Kansas and Oklahoma shoestring sands described by Bass and others shows sand-bar characteristics and origin more clearly than these Mississippian sand bars of Michigan.

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