Abstract: The Incised Valley and Valley Fill at Siccar Point
Timothy A. Cross, Peter W. Homewood
Siccar Point is one of the world's most famous and historically significant outcrops. Each year numerous geologists on pilgrimage visit the site where Hutton demonstrated his concept of unconformity by showing the angular discordance between underlying, near vertical strata and overlying horizontal strata. Hutton also noted that clasts derived from the truncated lower units were incorporated in the sediment of the overlying strata, further documenting the temporal separation of strata across the unconformity.
Siccar Point also is a world-class example of an incised valley and valley fill. Geologists can double their pleasure with views of the angular unconformity contained within the three-dimensionally exposed paleovalley filled by fluvial sediments of the Old Red Sandstone. The paleovalley is a low width-to-depth ratio, entrenched alluvial valley. One margin of the valley is very well exposed at Siccar Point. That margin is relatively steep (approx. 10 degrees), and fluvial strata onlap the valley wall with conspicuous discordance. Unidirectional, trough cross-stratified fluvial strata are granule, pebble coarse sandstones at the base and fine up within the valley. As is typical of entrenched alluvial valleys, in contrast with broad alluvial valleys, only nonmarine strata occur within th valley fill.
It seems fitting, in this decade of ingenuous rediscovery of paleovalleys, that stratigraphic preservation of the angular unconformity at this historically important outcrop is a consequence of containment within an incised valley. Perhaps fortuitously, the paleovalley at Siccar Point is not far from the first correctly and completely described incised valley and valley fill (Black, 1934).
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France