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Field meeting at Welwyn and Mardley Heath
FIELD
MEETING AT WELWYN MARDLEY HEATH.
AND
SATURDAY, 22ND APRIL, 1933. Director: S. W. WOOLDRIDGE, D.Sc., F.G.S.
A PARTY of members and friends, numbering about twenty, met at Welwyn North Station. They walked thence through Locksleys Park to Welwyn village, pausing en route to study the southward view. Attention Was called to the two dominant physiographic levels in the district-the 400 ft. platform represented by the summit plane of the Ayot and Burnham Green outliers and covered by quartzose shingle of the Pebble Gravel or Westleton type-and the flat floor of the Vale of St. Albans, where the deposits of the main local glaciation rested at about 200 ft. The walk was continued north-eastwards to the cross roads near the western end of Mardley Heath. Here there are pits in coarse gravel at about 380 ft. a.D. and over a small area, a mass of stony clay, closely resembling decalcified boulder-clay, rests on the gravels. Reasons were given for regarding this assemblage of deposits as representing an early glaciation of the district, distinct from that which produced the local Chalky Boulder-Clay and its gravels. The Mardley Heath deposits are separated from the latter by a distinct physiographic step of 150 ft. or more-presumably representing an interglacial phase of degradation. Moreover, it was noted that the contents of the Mardley Heath gravels differed from those of the lower glacial gravels, showing less variety. In particular the fragments of basic igneous rock and Jurassic debris are rare or absent, though " felsitic " rocks, both aphanitic and porphyritic, occur. The gravels of Mardley Heath are continued westwards on an upper bench or terrace; which follows the north side of the Vale of St. Albans, and they may be represented also near Roe Green and Radlett on the south of the Vale. The importance of the Mardley Heath exposures lies in the fact that here and there only is any deposit resembling boulder-clay associated with the gravels. It was suggested that both gravels and boulder-clay might prove to be of westerly provenance. The party returned to Welwyn for tea, and afterwards walked to a convenient viewpoint north-west of the village, from which the diversion of the Mimram by glacial drift, from its former course under Codicote Heath, could be studied. The opportunity was also taken of visiting the" Cemetery Pit" near by. The gravels here belong to the main or lower glacial series and the several respects in which they contrasted with the Mardley Heath Gravels were readily appreciated.
MEETING AT WELWYN MARDLEY HEATH.
AND
SATURDAY, 22ND APRIL, 1933. Director: S. W. WOOLDRIDGE, D.Sc., F.G.S.
A PARTY of members and friends, numbering about twenty, met at Welwyn North Station. They walked thence through Locksleys Park to Welwyn village, pausing en route to study the southward view. Attention Was called to the two dominant physiographic levels in the district-the 400 ft. platform represented by the summit plane of the Ayot and Burnham Green outliers and covered by quartzose shingle of the Pebble Gravel or Westleton type-and the flat floor of the Vale of St. Albans, where the deposits of the main local glaciation rested at about 200 ft. The walk was continued north-eastwards to the cross roads near the western end of Mardley Heath. Here there are pits in coarse gravel at about 380 ft. a.D. and over a small area, a mass of stony clay, closely resembling decalcified boulder-clay, rests on the gravels. Reasons were given for regarding this assemblage of deposits as representing an early glaciation of the district, distinct from that which produced the local Chalky Boulder-Clay and its gravels. The Mardley Heath deposits are separated from the latter by a distinct physiographic step of 150 ft. or more-presumably representing an interglacial phase of degradation. Moreover, it was noted that the contents of the Mardley Heath gravels differed from those of the lower glacial gravels, showing less variety. In particular the fragments of basic igneous rock and Jurassic debris are rare or absent, though " felsitic " rocks, both aphanitic and porphyritic, occur. The gravels of Mardley Heath are continued westwards on an upper bench or terrace; which follows the north side of the Vale of St. Albans, and they may be represented also near Roe Green and Radlett on the south of the Vale. The importance of the Mardley Heath exposures lies in the fact that here and there only is any deposit resembling boulder-clay associated with the gravels. It was suggested that both gravels and boulder-clay might prove to be of westerly provenance. The party returned to Welwyn for tea, and afterwards walked to a convenient viewpoint north-west of the village, from which the diversion of the Mimram by glacial drift, from its former course under Codicote Heath, could be studied. The opportunity was also taken of visiting the" Cemetery Pit" near by. The gravels here belong to the main or lower glacial series and the several respects in which they contrasted with the Mardley Heath Gravels were readily appreciated.