Key Feature:
HORNFELS: a fine-grained metamorphic rock that was originally a sedimentary rock. Rocks such as mudstone, shales and limestone come into contact with an igneous (magma) intrusion at shallow depths therefore are formed under minimal pressure. The intense heat bakes the rock making it extremely hard, splintery, tough, durable, and dense.
Background:
We acknowledge the Ngunnawal people as traditional custodians of this region. The Red Hill Campsite (Reference Point: R0) was used until the late 1940s.
The first Europeans settled in the ACT in the 1820s. Red Hill Campsite remained one of very few areas of Crown Land near the Molonglo River until the 1860s. The marshy nature of the ground made this an unlikely area for pasture and therefore ill-suited to European farming practices.
Geology:
Hard hornfels in the Red Hill area is the main rock type known as the Yarralumla Formation. Sediments were deposited under shallow marine conditions during the mid-Silurian period approximately 430 million years ago during a pause in volcanic activity. The volcanic intrusion that impacted these deposits is dated at about 425 million years ago and is known as the Federal Golf Course Tonalite. Before becoming metamorphosed from the extreme heat of the igneous intrusion these rocks resembled those of the Deakin Anticline where you can find a related EarthCache - GC3M6EY.
The Red Hill outcrop mostly contain hornfels called calc–silicate hornfels that arise from the thermal alteration of impure limestone. It weathers orange-brown, and cavities have formed where the more soluble parts of a metamorphosed nodular calcareous mudstone have weathered out. At Virtual Stages 02 and 03 this type of weathering is evident. Hornfels are otherwise highly resistant to erosion with dark-grey to light-grey banding indicating the original bedding deposits. At Virtual Stage 02 you are standing at the centre of a disused quarry.
Image 2: Hard Hornfels Red Hill - up close showing the distinct layers of bedding deposits. Photograph taken: August 2022.
Access:
Parking is available at the far end of Red Hill Lookout. The walk to Virtual Stage 01 is on track. The short walk down to Virtual Stages 02 & 03 is down a short steep slope to the old hornfels quarry.
Logging Requirements:
To log this cache you will need to address the following 4 points (3 questions and 1 mandatory photo). Send your answers via 'Message Center' to the CO as the preferred method. You can log this cache with the required photo immediately. If there are issues with your answers or photo, you will be notified.
Questions / Photo:
Virtual Stage 01: Extruded Hornfels Boulders.
1. What is the average width in millimetres of the dark-grey strips in the boulders just off trail, and what do you think the grey bands represent?
Virtual Stage 02: Hornfels Quarry: Boulders and Rocks.
2. There is one large hornfels boulder, and many small hornfels rocks found here. Pick up one of the small rocks. What do you notice about the weight of the rock, and why do you think is - light OR heavy - choose one option and explain?
Virtual Stage 03: Hornfels Quarry: Trackside.
3. On the southern side of the large boulder, why do you think it is orange-brown in colour, and that the banded strips are not visible?
4. Post a photo of yourself, or something that identifies you, with the Hornfels boulder strips visible on the northern side (other side) of the boulder.
Consider posting additional photos to encourage others to come to this part of Red Hill Lookout.