Boulder Reef

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Figure 2.1 - Shaded relief map of Boulder Reef, N. Lake Michigan. The rough relief in the southern, and shallowest, portion of the reef is covered with gravel, cobbles and boulders (Fig 2.2, Edsall et al., 1989 and video data) with rare patches of sand. The coarse bottom material is roughly organized into a boxwork of highs and lows less than a meter high. The arcuate ridge at the south end of the reef is likely a remnant depositional glacial feature from a small lobe of ice moving from the north. The coarse texture material extends to beyond the depths of the survey to the west and south, but transitions to a sand substrate to the east and north. (Video observations by Greg Kennedy, personal communication) The strong east west lineations in depths of 15-20 m in the northern half of the image could be related to either glacial movement but this is transverse to movement suggested by the ridge to the south, or to older glacial events or to the extrapolated underlying Devonian, Bois Blanc Formation (Fig.1). These lineations are drapes overlain by sediment (sand with smooth) morphologies. Clean sand was sampled in this part of the ridge. Underwater observations of Somers (1968) reports, "till generally is covered with sand, gravel, cobbles or boulders" from a dive near the south end of this reef. Note: The faint N-S lineations seen in the central part of the image are artifacts of the data gathering flight lines. (See full-size JPEG--328 kb)

Thumbnails

	Figure 2.2 - Oblique view looking northeast toward the straits of Mackinac. The relatively steep slopes of the south end of the ridge and the different rough morphologic character of the cobble and gravel substrate fill the foreground. A small depositional apron of sand (video data) is forming off the southeast part of the reef. Distance across the bottom of image is about 2.9 km. (JPEG 393 kb)
	Figure 2.3 - Oblique view of Boulder Reef looking southwest emphasizes the east-west lineations and smoother morphology of a sand draped surface on the northern part of the reef. A "window" of gravel/cobbles is likely in the foreground at the northern edge of the map. Distance across the bottom of image is about 3.0 km. (JPEG 448 kb)
	Figure 2.4 - Surficial geology and habitat of the southwestern part of Boulder Reef interpreted from side-scan sonar mosaic and video and mosaic (modified from Edsall et al., 1989). (JPEG 173 kb)

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