The Goring Gap is the narrow valley, occupied by the River Thames, between the Chiltern Hills and the Berkshire Downs. It is approximately 10 miles (16 km) upstream of Reading and 27 miles (43 km) downstream of Oxford. The river here forms the county boundary between Berkshire and Oxfordshire. Half a million years ago the chalk hills at Goring formed a continuous escarpment, which directed the Thames to flow northeastwards to reach the North Sea near Ipswich. During the last ice age, an ice sheet blocked the river's downstream course through Hertfordshire, forcing the water to back up and to form a large lake.
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