The Straits of Mackinac are a complex of simple straits situated at the outlet of Lake Michigan. Water movements through the Straits region normally are complicated by the actions of winds and seiches of the two lakes but, on a long-term basis, the outflow of Lake Michigan must pass through the Straits into upper Lake Huron. A general west-to-east set of current through the Straits is reflected in the distribution of silt. Bottom sediment samples from a 640 square mile area of upper Lake Michigan, the Straits, and upper Lake Huron show a paucity of silt in upper Lake Michigan and in the Straits, and an apparent accumulation of silt in uper Lake Huron where pronounced increase in cross-sectional area appears to result in decreased current velocity. The few current measurements in the Straits indicate that current velocities sufficient to transport silt are common there; under high winds or strong seiche action strong currents are encountered in the Straits and are considered as probably being of sufficient velocity to erode silt. Bottom currents in upper Lake Huron, measured after several days of west winds, were only strong enough to transport silt; under lighter winds or shifting winds bottom currents slow enough to allow deposition of silt appear to be very probable. Sandy sediments predominate over most of the study area and are the characteristic sediments of depths up to 100 feet. Silty sediments are dominant in the upper end of Lake Huron, and clays or tills are characteristic of the floors of the Stanley River valley and of the elongate deepest portion of South Channel. From Upper Lake Michigan to a point north of Mackinac Island the Stanley River valley is floored for the most part with sandy red clay which frequently contains pebbles and appears to be Valders till. This is also the condition in the elongate deepest portion of the South Channel. Those parts of the Stanley valley lying east of Mackinac Island and north of Bois Blanc Island appear to have received wave-worked materials from the island and to have been partially filled. A similar encroachment may be taking place in the deep portion of South Channel, except that in this case the encroaching sediments seem most likely to coming from the mainland, and the causative agent appears more apt to be current scour. Landward extensions of the 50- and 100- foot isobaths were found off Big Stone Bay, St. Martin Bay, and Marquette Bay. The configuration of these extensions suggests that they may be submerged tributaries to the Stanley valley. Three similar eastward extensions of the 100-foot isobath were present in the west end of South Channel. In none of the six cases was there a corresponding extension of the 150-foot isobath, a circumstance which might be interpreted as indicating a water-level above the present 150-foot depth at some low-water stage (possibly post-Stanley?) when active tributaries may have existed in the extensions of the upper isobaths. In South Channel and east of Bois Blanc Island the distributions of red clays and tills suggest a Valders ice shadow which resulted in failure to deposit Valders till on the older grey Port Huron till. Fairly extensive deposits of Valders till and clay along the south side of South Channel in the vicinity of Cheboygan are suggestively close to the position of the Black Lake Valders ice block, and might be slump or outwash materials from that block.