Get­ting to the bot­tom of the Ox­ford sink­hole mys­tery

The Amherst News - - OPINION - Alan Wal­ter Alan Wal­ter is a re­tired pro­fes­sional en­gi­neer liv­ing in Ox­ford. He was born in Wales and worked in Hal­i­fax. He spends much of his time in Ox­ford, where he op­er­ates a small farm. He can be reached at alan­wal­ter@east­link.ca.

Did You Know

The Car­bonif­er­ous Pe­riod be­tween 300 and 350 mil­lion years ago was a very busy time in what we now know as Cum­ber­land County. The ear­li­est species of plants and an­i­mal life on the planet were flour­ish­ing here and would con­trib­ute to Jog­gins Fos­sil Cliffs’ un­ri­valled fos­sil record.

Our planet’s con­ti­nents were also con­verg­ing at that time to form Pangea, a sin­gle su­per­con­ti­nent, on which our prov­ince-to-be could be found near the equa­tor.

How­ever, this was no “ocean play­ground” - all but the high­lands in Nova Sco­tia, and Prince Ed­ward Is­land were sub­merged in an an­cient body of sea wa­ter that was named by ge­ol­o­gists as the Wind­sor Sea, af­ter the Wind­sor area of our prov­ince.

Sea lev­els pe­ri­od­i­cally rose and fell dur­ing the 15 mil­lion years that the Wind­sor Sea ex­isted. Iso­lated bays, while pe­ri­od­i­cally cut off from the sea, were reg­u­larly re­plen­ished with in­com­ing sea wa­ter; and vast de­posits of min­er­als, such as gyp­sum and salt, were laid down over that pe­riod through evap­o­ra­tion.

The Cana­dian Salt Co. Ltd op­er­a­tion in Pug­wash now prof­its from min­ing such a salt de­posit, large enough for the fu­ture life­span of the min­ing op­er­a­tion to be es­ti­mated at more than 100 years; and the salt it­self is ap­pro­pri­ately branded and mar­keted as “Wind­sor Salt.”

As for the Town of Ox­ford’s sink­hole sit­u­a­tion, it is likely a re­sult of this same al­ter­nat­ing flood­ing and evap­o­ra­tion process cre­at­ing de­posits of gyp­sum be­neath the town, in as yet un­known quan­ti­ties.

The word gyp­sum is de­rived from the Greek word “gyp­sos” for “plas­ter”, and gyp­sum has many uses to­day in con­struc­tion as a mor­tar and in dry­wall fab­ri­ca­tion. As a min­eral de­posit it be­haves as a soft rock that will even­tu­ally dis­solve in ground­wa­ter. In Ox­ford a large quan­tity must have dis­solved cre­at­ing a cav­ity which be­came a large sink­hole in the Ox­ford Lions Park.

What hap­pened in that park would come as no great sur­prise to ex­perts in the field, since an on-line ge­o­log­i­cal map of Nova Sco­tia clearly in­di­cates the like­li­hood of gyp­sum de­posits in the Ox­ford area. And gyp­sum sink­holes are not an un­com­mon oc­cur­rence in the prov­ince, al­though this one is al­ready quite spec­tac­u­lar.

Amy Tiz­zard, a ge­ol­o­gist with the Nova Sco­tia De­part­ment of En­ergy and Mines, ad­vised it was too early to tell how big the Ox­ford sink­hole will get, say­ing “sink­holes are un­pre­dictable in their na­ture so we can’t rule any­thing out at this point.”

She added, “we want to gather as much in­for­ma­tion as pos­si­ble around the sink­hole so we’re look­ing at dif­fer­ent in­ves­tiga­tive tech­niques that in­cludes some high pre­ci­sion sur­vey­ing to mon­i­tor dif­fer­ences in el­e­va­tion and us­ing ground-pen­e­trat­ing radar which can help im­age the sub­sur­face and get a bet­ter idea of the se­ri­ous­ness of the prob­lem.”

The prov­ince has also asked the fed­eral govern­ment for as­sis­tance and ad­di­tional geo­phys­i­cal scan­ning equip­ment is be­ing dis­patched to the site.

The Town of Ox­ford will be able to rest eas­ier when the in­ves­tiga­tive re­sults hope­fully in­di­cate a re­stricted, man­age­able sit­u­a­tion; and the Lions Park fa­cil­ity can re­sume its im­por­tant com­mu­nity role fol­low­ing some al­beit costly repa­ra­tions.

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