These formed as the result of large volcanic explosions.
The good agreement with the tree ring curve in the interval from 7000 to 11450 cal y BP (cal y indicates calendar year) proves the annual character of the laminations.
The ash layer of the Vasset/Killian Tephra (Massif Central, France) is dated at 8230±140 C y BP and 9407±44 cal y BP.
(In practice there may be technical difficulties, but the principle is straightforward enough.) Of course, this only works if there is still a source of sediment, so that we can identify this year's varve and know which year we're counting from.
Once the source of sediment is cut off, the link with the present is severed, and unless we could find some other method to place an absolute date on one of the varves, the only thing we can tell from them is the difference in age between two varves, but not how old either of them is. We can count back thousands or tens of thousands of years, but as we shall see in later articles this is only a short span of time in proportion to the much longer history of the Earth.
One place were varves have been studied for decades is below a deep lake in Japan: Lake Suigetsu.
Here a varve chronology stretching back well over 50,000 years has been established.The evidence from varves presents a formidable challenge to young-earth creationists (YECs) and their assertion that scientific evidence, properly interpreted, points to a young earth.Though a well-worn example, this recent work pushing the varve chronology to close to 60,000 year bears reviewing in light of how YECs have responded in the past to this challenging data.An important summary of the significance of this varve chronology can be found in an open access article from magazine here.For several decades, varve chronologies have been featured as clear evidence of an ancient Earth.Below I introduce you to the varve deposits in Japan and then review how young earth creationists have responded.