Based on 'Rock glazes of NZ' Minna Bondy
Sample 79. Pukaki River Sand
Almost certainly derived from Greywacke and possibly schist. Much very fine-grained material ("rock flour ") present.
After a nights stay in Twizel at minus 8 degrees the road between Twizel and Lake Pukaki was chilly and covered with a rare with hoar frost. Hoar frost is a frozen fog that forms a spikey crystal formation when it comes in contact with an object. Visibility of the lakes and rivers was limited but what could be seen was incredible.
It was like searching for an object on the moon as the environment was so foreign, you could only see for 20 metres or so but I soon found a beach on the lakeside to collect sand and finely ground rock dust. Lake Pukaki is fed from Glaciers so the rock is already finely ground stone from glacial movement. It gives the lake its bright blue milky appearance as its particles reflect light...none of which could be seen the day I was there, and it was way too cold to hang around. The rocks on the foreshore were also the same green-blue as the lake when cracked.