Croham Hurst is a geological anomaly, as you'll see if you go walking on it. Instead of earth or rock, the ground underfoot often consists of a carpet of smooth pebbles.
Hundreds of thousands of years ago, much of Surrey was covered in pebbles, the remains of a vast prehistoric beach that stretched from Surrey to Artois in France. These pebbles have mostly now washed away, but in some places, such as our own dear Hurst, they remain, bound together by a natural concrete.
In geological terms, Croham Hurst is a belt of Blackheath Pebbles lying on a base of eroded Thanet sand, which in turn rests on chalk. If you have a garden nearby, you'll realize how very close to the surface the chalk mostly is. In the case of the Hurst, though, iron deposits in the sand have bound sand and pebbles tightly together and formed a hill. This is also why so many of the pebbles have a rusty, brownish tinge. The pebble/sand concrete is called 'pudding stone'.
This is why the oak trees that grow all over the Hurst have such pleasantly twisted and climbable shapes; pebbly ground keeps them smallish and makes them stretch their roots and branches out horizontally, making the Hurst a wonderland of climbable trees, from the point of view of the under-tens. It's very likely that this pebbly geology is the reason Croham Hurst, Blackheath, and some of the other pebbly hills in Surrey still have trees on, including very old trees -- they weren't cut for timber because they don't grow straight. This doesn't apply around the base of the Hurst, where the soil is deep and straight beech trees seem to have been planted some time in the past.
The pebbly terrain of the Hurst is pleasant to behold and to walk on, but very vulnerable to erosion. At the west edge of the Hurst, the steepest and most-used path up the hill is starting to gully a bit. It's therefore important not to treat steep paths as mountain bike tracks, lest one day we find the Hurst adopting a lower, flatter, shape reminiscent of a sat-upon jelly.