|Hillgrove Monzogranite on the Waterfall Way|
According to the Australian Stratigraphic Names Database the Hillgrove Monzogranite was until recently known as the Hillgrove Adamellite (Adamellite being the outdated synonym for Monzogranite). It was previously classified as part of the Hillgrove suite which in turn is part of the Hillgrove Supersuite. However, based on geochemical properties (and possibly just to confuse people) the Hillgrove Monzogranite is no longer considered part of the Hillgrove suite instead just being a member of the Hillgrove supersuite! However, it is clearly one of the S-type plutonic rocks collectively known as the New England Batholith (Bryant et al 2003).
Monzonite is unsurprisingly the dominant rock type of the Hillgrove Monzonite. It is an S-Type granite (derived from melted sedimentary rock). It is comprised mainly of quartz and feldspars (roughly equal potassium feldspar and sodium-calcium Feldspar), quartz, biotite mica and hornblende. The biotite often shows a foliation, which is a preferred alignment in the rock. The age of the Hillgrove monzogranite is estimated at between around 270 to 290 million years. To my knowledge, the age has not been directly measured but instead is based on its relationship to the surrounding rocks with their either calculate or approximate ages.
The landscape formed by the Hillgrove Monzogranite is one of my favourites. It forms a relatively large plateau which contains low rolling hills and lovely boulder outcrops. These outcrops often form lovely torrs (see pictures) formed by “onion-skin” weathering. Water enters cracks in the rock and during winter this freezes and expands gradually wedging the layers off the boulder. This is correctly termed frost wedging.
The Bakers Creek gorge has cut into some of the unit near the Hillgrove area but overall the appearance of the country is quite gentle. The rock unit extends a long distance from the location of Argyle in the west almost to Chandler Gorge in the east. The Waterfall Way (Armidale-Dorrigo Road) crosses in and out of the Hillgrove Monzogranite and Girrakool Beds into which it has intruded. Therefore it is an easy stop on the road when travelling this route.
The soils are sandy and not very fertile leading to an area used for cattle and sheep grazing on native and improved sown pastures. The forest is an open dry sclerophyll snow-gum type bush which is one of the typical environments of the New England high country. I love the appearance of this country. It is the quintessential high-lean New England landscape.
*Bryant, C.J., Chappell, B.W. & Blevin, P.L. 2003. Granites of the Southern New England Orogen. Abstracts of the Ishihara Symposium: Granites and Associated Metallogenesis. GEMOC, Macquarie University