Excursion Report - Croft Quarry

Excursion Report - Croft Quarry, Leicestershire,

September 9^th 2006

Our penultimate trip of the 2006 field season was to Croft Quarry in Leicestershire. Twenty one members including colleagues from the Warwickshire Geological Conservation Group driven up by Martyn Bradley assembled at the offices of Bardon Aggregates at the quarry. After the mandatory ‘signing-in’ and an inspection of motive power on the internal railway system by certain members, we travelled in a convoy of minibuses courtesy of the Quarry and Martyn Bradley. Our leader for the day was Dr John Carney from the BGS at Keyworth, and quarry employees Bill Rathbone and Martyn Williams escorted us. John had prepared a first class handout detailing the geology of the Quarry and relevant research into its origins, from which the details given in this account are taken.

Croft Hill and Quarry constitute an exposure of Caledonian intrusive rocks. The rock is calc-alkaline in nature and together with outcrops at Stoney Stanton and Sapcote constitutes the South Leicestershire Diorites. First stop was mid-way down the Quarry where we stopped for a brief introduction from John Carney and to examine the rocks at that point. Interesting features of these rocks are the microdiorite xenoliths. We drove further into the quarry hoping to see veins and cavities of calcite zeolite and laumontite, analcime and chalcopyrite. But although we could see cavities at the base of the faces, the areas were flooded and we were unable to get close and collect - another time perhaps. Some skilful driving by the quarry employees and Martyn guided us up past the massive faces of quartz diorite, generally pale grey in colour and with orthogonal jointing. Bill Rathbone explained that a process of back-to-back blasting creates a clean safe face above the roadways. Next stop was to examine the Triassic unconformity towards the top of the quarry. The unconformity face is irregular, with deep, v-shaped palaeovalleys. The sequence overlying the unconformity is red mudstone interbedded with grey dolomitic siltstones. These constitute the Mercia Mudstone Group and were laid down under arid conditions punctuated by short-lived flooding episodes represented by the dolomitic siltstones. Compaction of the Mercia Mudstone appears as sagging above the deepest parts of the palaeovalleys and this structure is known as a catenary dip. As time was limited we made our way towards the quarry exit, stopping to examine sheets of darker grey quartz-diorite in the Quarry wall, between 1 and 3 metres wide. These extend up the face for tens of metres and occurred as part of the intrusion. The stone is much in demand for road surfacing and building work and whilst much is transported by road there are regular rail runs to Bow Freight terminus in London. The trip wound up at approximately 12:45 pm and after thanking John, Bill and Martyn for an excellent excursion we departed.

Not wishing to curtail a fine day too soon, a select party of enthusiasts made their way to the Heathcote Arms in Croft village, where a jolly lunchtime ensued.

Helen Jones & Andrew Swift