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304 Bays. After a contour map of the bedrock surface had been prepared on the basis of that survey, three holes were drilled to bedrock; one of these was near the deepest part of the sinkhole, one on the westerly slope of the depression, and the third, in line with the other t+vo, w•as slightly east of the ++•esterly shore of the pond occupying the sinkhole azea. The results of the drilling indicated that the seismic survey had overestimated the depth to bed- rock by sixty to eighty feet within that part of the sinkhole azea where the measured depth was 92 to 98 feet. After the results of the borings became available, the results of the seismic survey were re- computed. Figure 8, an east-west cross section through the sinkhole, shows the results of soundings in the o,.r.•., u... ,..,~. .p ~- ~~ ,~ i ~- --; I .- --- ...x..,.-,.,,..r...... ,..,,,., rru I I `~`.' Figure 8. East-West sec[ion through sinkhole. sinkhole pool, the depth to bedrock determined at three points by borings, and the revised results of the seismic survey. According to the seismic sur- vey, the Pleistocene deposits +vere as much as 25 feet thicker beneath the easterly part of the depres- sion than beneath the westerly part, where the ob- served thickness of 92 to 98 Eeet is close to the values recorded in the logs of brine wells in the vicinity of the sinkhole (85 to 94 feet). In the absence of additional borings, the accuracy of the recomputed results of the seismic suney cannot be regarded as established. Within the period of four months following the formation of the sinkhole, two sets of settlement observations +vere carried out. Both of these sur- veys included reference points in the area located to the northwest of the sinkhole, as well as those in the sou[hvvestcrly section which had not been sur- veyed since 1950. The results of the levelling showed that rather irregular movcmcttts of small magnitude had taken place +vithin [his period. ~rinelield Subsidence et Windsor, Ontario Some points went up whereas others settled. In general, additional settlements of as much as one inch had taken place in the zone adjacent to the sinkhole pond; settlements of other parts of the brine field were within the limits of error of the survey. Because of a change of o++itership, observation of reference points in the northwesterly part of the subsiding area was discontinued after 1954. In the southw~esterl}' quadrant, ho+vever, the Canadian Salt Co. (J.D. !\9air, personal communication, 1969) carried out annual suneys of some thirty reference points, about half of which had been in existence since 1948 (Fig. 9). Most of the points under observa[ion have settled at average rates of less than one-fourth inch per tear since 1954, and the se[[lemrnt of mam of them appears to have ceased altoge[her. The settlement of points located at a distance of 200 to 500 feet from the sinkhole area is scarcely perceptible. Their average annual rate of settlement over [he 14-year period follow- ing sinkhole formation does not exceed about one- seventh of the average annual rate observed from 1948 to 1950 (the only years for which data are available for this part of the subsiding area), and it therefore appears that the formation of the sink- hole coincided with the end of the process of ++ide- spread, accelerating subsidence which preceded it. 1 2. ~~ 1.0 OS• 26 O.I ~-4.01.2 ~o 7. <.5 ~ b 5:.3 5 3.6•~•~4.0 4~ 6.5 6.2 ' ORIG INdL `~~ •66.0 % EOGE OF • 3I.5 ~ ~, 61N KlIOLE '. ~~ •130 2 ~ t:~~~. re6e1 2.6 ••125_ _ ~ao a3 28 30 -2W ff- PL411T NO'n Tr Figure 9. Scttl<ncn is in inches, 1951 to 1969.