The programme indicated visits to 7 localities plus 1 optional today, but due to the interest shown we only managed to visit the 4 most important ones.

Huttenberg Sand Pit

Our first visit was to the Huttenberg Sand Pit in the rim of the Huttenberg crater. This was a very interesting location as the Huttenberg tephra is very poorly exposed, with only 3 exposures in the area.

The pyroclastic rocks were cross bedded throughout, mainly very well bedded in massive layers with flat tops and bottoms; indicating a horizontal turbulent flow, the direction of which was not evident as no perpendicular face available to view. Flow was, in fact, west to east.

The pyroclastic flow was rich in ash with the fine ash having risen several kilometres and drifted in the wind so that it was decoupled from the main flow. Changes in eruption, or transport, caused change in the layer grain size from lapilli bombs to ash in every layer. Magma erupted was crystal rich (20%) with phenocrysts of 2 feldspars (anorthoclase & plagioclase), hornblende (dark needle shaped) and augite (shorter, dark green/black). With 10-20% vesicles, this was not a pumice but indicated a water quenching of the magma as it erupted, rather than magmatic degassing.

The Huttenberger Sand Pit

There had been regional metamorphism of 3 groups of country rock:- Devonian slates, Devonian sandstones, and low grade phyllite & mica schists.

Contact metamorphism changed the mineralogy of the regionally metamorphosed rock so that it crumbled between finger & thumb, as the rock became 'sandy'. This was caused by the magma chamber at the phyllite level, or at the boundary of the unmetamorphosed Devonian rocks. The country rock, rich in silica, was taken up by the magma, i.e. phonolite magma erupted, reacted with country rock taking up silicate, and became a trachyte.

The phonolite with large crystals of mafic composition indicates it was not highly evolved, so from the deepest level of the magma chamber; the end of the eruption, 215,000 years ago.

Dachsbusch Quarries

The caldera at this location was too large for the Huttenberg Sand Pit deposits so it is probable that these de-posits were from an earlier eruption, but the expected ring dykes have not been found.

Moderate to poorly sorted clasts were present in a well bedded pumice which did not have any cross bedding. Very few crystals but many vesicles, and the rock were whiter than expected. The occasional large holes in the face were from tree branches & trunks which had been dragged along with the flow and now long gone, although they possibly changed to charcoal before disappearing.

Basalt tuff was present but no Devonian rocks. Huttenberg tephra is always found at the base of scoria deposits (lava lumps) and at a second locality in the quarry, the lowest unit was the scoria; horizontally bedded, poorly sorted ash and basalts (ash -lapilli in size) lay above, showing well bedded material with some low angle cross bedding.

This was a surge deposit of magma; mainly basalt containing olivine & clinopyroxene minerals, Devonian sandstone & slates, and pieces of Huttenberg tephra. Evidence indicated the basalt had been water quenched and as tephra pieces present, the Huttenberg eruption was older (215,000 compared to 150,000 ya).

Severe folding

These deposits were parallel to the flank & cone and during the ice age (sometime bet. 215,000 and 150,000 ya) the structure developed in a cold, tundra vegetated permafrost period, with freeze/ thaw to several metres depth. This did not destroy the bedding and, when no ice above, movement downwards of the plastically composed rock, folded the bedding over, creating a very unusual structure. A classic example of 'plastic deformation'.

Rothenberg Quarry

The locality had several eruption centres and in the rubble we saw clinopyroxene crystals up to 10mm diameter which reflected blue/green in the strong sunlight. The basalt contained 10% clinopyroxene crystals and a little olivine and biotite mica (mg rich). We were looking at a massive structureless basalt which had a 'welded core' as a mixture of basalt bombs from nearby vents welded together in a very hot, explosive eruption and fell back (about 200000 ya).

At the base of the volcano we saw the weathered surface of the older strata, and the volcano plug with baked tuff alongside.

The baked rock was from a previous eruption and the columner jointing was into the face; indicating that cooling was vertical. The plug was in 3 distinctive bands; greenish at the centre indicating hot rock not exposed to air but to volatile vapour & gases, then reddish…. hot and exposed to air, and black on the outside indicating exposure to air when cold.

Leader on sandy cliff

Eppelsberg Quarry

Last visit of the day but notices stated 'entry prohibited' as this was a popular tourist site. The restriction did not deter as 10 information boards had been erected along a footpath which rose above, and overlooked, the quarry. Unfortunately, wording was in German but the drawings were colourful and understandable.

The eruption occurred about 200,000 ya and the clear, sunlit rock face showed evidence of 6/7 eruptions, with the volcano to the west. It was of basalitic composition with an ash fall on top.

As with the other days, this was another hot, dusty, unshaded day, but tremendously interesting and informative, which made the OU Summer School on the 'hard rock' - course pale into insignificance. Ex-students wished they had brought along their S339 books, as this day really brought them to life.

Dave Williams (Branch Organiser -Yorkshire)