Unity中如何让物体在高地（山丘、山地）生成？

我制作的场景包含山脉、山谷与山丘，需要实现物体仅在山谷及低地区域生成，无法在山丘上生成，以下是对应的Unity实现代码：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ResourceGenerator : MonoBehaviour
{
    [Header("生成设置")]
    public GameObject resourcePrefab;
    public float spawnChance;

    [Header("射线检测设置")]
    public float distanceBetweenCheck;
    public float heightOfCheck = 1000f, rangeOfCheck = 30f;
    public LayerMask layerMask;
    public Vector2 positivePosition, negativePosition;

    private void Start()
    {
        SpawnResources();
    }

    void SpawnResources()
    {
        for(float x = negativePosition.x; x < positivePosition.x; x += distanceBetweenCheck)
        {
            for(float z = negativePosition.y; z < positivePosition.y; z += distanceBetweenCheck)
            {
                RaycastHit hit;
                if(Physics.Raycast(new Vector3(x, heightOfCheck, z), Vector3.down, out hit, rangeOfCheck, layerMask))
                {
                    if(spawnChance > Random.Range(0f, 101f))
                    {
                        Instantiate(resourcePrefab, hit.point, Quaternion.Euler(new Vector3(0, Random.Range(0, 360), 0)), transform);
                    }
                }
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Florens Kosicek