To the north of the city a ridge capped by the mid-to-lower basin's residual sands known as the Bagshot formation forms high ground (in places around 130m) including Hampstead Heath and Highgate Hill. The ridge continues briefly eastwards over the London clay to form Crouch Hill and Queen's Wood. Immediately south, fingers of the ridge run down towards Primrose Hill and Parliament Hill. This ridge is a surviving area of Tertiary rocks younger than the London Clay, surrounded by former routes of the Thames where much younger deposits overlie the clay. Smaller outliers of such Tertiary high ground exist to the west of the main ridge especially relatively lowly Hanger Lane; the knoll Harrow Hill, and smaller Horsendon Hill where the Claygate Beds (the top of the London Clay formation) are capped by much younger gravels deposited by watercourses.