How can i prevent from moving the camera view under the terrain ?

I'm using this mouseorbit script attached to a camera. The problem is when i move the camera with the mouse and rotating it so the camera is under the terrain. I want that when it get to the terrain height then stop don't move down i mean don't get to this view under the character maximum to be in the terrain height..

To stop on terrain height i mean something like that when it's getting to this:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class MouseOrbit : MonoBehaviour
 {
     /* These variables are what tell the camera how its going to function by
       * setting the viewing target, collision layers, and other properties
       * such as distance and viewing angles */
     public Transform viewTarget;
     public LayerMask collisionLayers;
     public float distance = 6.0f;
     public float distanceSpeed = 150.0f;
     public float collisionOffset = 0.3f;
     public float minDistance = 4.0f;
     public float maxDistance = 12.0f;
     public float height = 1.5f;
     public float horizontalRotationSpeed = 250.0f;
     public float verticalRotationSpeed = 150.0f;
     public float rotationDampening = 0.75f;
     public float minVerticalAngle = -60.0f;
     public float maxVerticalAngle = 60.0f;
     public bool useRMBToAim = false;
 
     /* These variables are meant to store values given by the script and
      * not the user */
     private float h, v, smoothDistance;
     private Vector3 newPosition;
     private Quaternion newRotation, smoothRotation;
     private Transform cameraTransform;
 
     /* This is where we initialize our script */
     void Start()
     {
         Initialize();
     }
 
     /* This is where we set our private variables, check for null errors,
      * and anything else that needs to be called once during startup */
     void Initialize()
     {
         h = this.transform.eulerAngles.x;
         v = this.transform.eulerAngles.y;
 
         cameraTransform = this.transform;
         smoothDistance = distance;
 
         NullErrorCheck();
     }
 
     /* We check for null errors or warnings and notify the user to fix them */
     void NullErrorCheck()
     {
         if (!viewTarget)
         {
             Debug.LogError("Please make sure to assign a view target!");
             Debug.Break();
         }
         if (collisionLayers == 0)
         {
             Debug.LogWarning("Make sure to set the collision layers to the layers the camera should collide with!");
         }
     }
 
     /* This is where we do all our camera updates. This is where the camera
      * gets all of its functionality. From setting the position and rotation,
      * to adjusting the camera to avoid geometry clipping */
     void LateUpdate()
     {
         if (!viewTarget)
             return;
 
         /* We check for right mouse button functionality, set the rotation
          * angles, and lock the mouse cursor */
         if (!useRMBToAim)
         {
             /* Check to make sure the game isn't paused and lock the mouse cursor*/
             if (Time.timeScale > 0.0f)
                 Cursor.lockState = CursorLockMode.Locked;
 
             h += Input.GetAxis("Mouse X") * horizontalRotationSpeed * Time.deltaTime;
             v -= Input.GetAxis("Mouse Y") * verticalRotationSpeed * Time.deltaTime;
 
             h = ClampAngle(h, -360.0f, 360.0f);
             v = ClampAngle(v, minVerticalAngle, maxVerticalAngle);
 
             newRotation = Quaternion.Euler(v, h, 0.0f);
         }
         else
         {
             if (Input.GetMouseButton(1))
             {
                 /* Check to make sure the game isn't paused and lock the mouse cursor */
                 if (Time.timeScale > 0.0f)
                     Cursor.lockState = CursorLockMode.Locked;
 
                 h += Input.GetAxis("Mouse X") * horizontalRotationSpeed * Time.deltaTime;
                 v -= Input.GetAxis("Mouse Y") * verticalRotationSpeed * Time.deltaTime;
 
                 h = ClampAngle(h, -360.0f, 360.0f);
                 v = ClampAngle(v, minVerticalAngle, maxVerticalAngle);
 
                 newRotation = Quaternion.Euler(v, h, 0.0f);
             }
             else
             {
                 Cursor.lockState = CursorLockMode.Confined;
             }
         }
 
         /* We set the distance by moving the mouse wheel and use a custom
          * growth function as the time value for linear interpolation */
         distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel") * 10, minDistance, maxDistance);
         smoothDistance = Mathf.Lerp(smoothDistance, distance, TimeSignature(distanceSpeed));
 
         /*We give the rotation some smoothing for a nicer effect */
         smoothRotation = Quaternion.Slerp(smoothRotation, newRotation, TimeSignature((1 / rotationDampening) * 100.0f));
 
         newPosition = viewTarget.position;
         newPosition += smoothRotation * new Vector3(0.0f, height, -smoothDistance);
 
         /* Calls the function to adjust the camera position to avoid clipping */
         CheckSphere();
 
         smoothRotation.eulerAngles = new Vector3(smoothRotation.eulerAngles.x, smoothRotation.eulerAngles.y, 0.0f);
 
         cameraTransform.position = newPosition;
         cameraTransform.rotation = smoothRotation;
     }
 
     /* This is where the camera checks for a collsion hit within a specified radius,
      * and then moves the camera above the location it hit with an offset value */
     void CheckSphere()
     {
         /* Add height to our spherecast origin */
         Vector3 tmpVect = viewTarget.position;
         tmpVect.y += height;
 
         RaycastHit hit;
 
         /* Get the direction from the camera position to the origin */
         Vector3 dir = (newPosition - tmpVect).normalized;
 
         /* Check a radius for collision hits and then set the new position for
          * the camera */
         if (Physics.SphereCast(tmpVect, 0.3f, dir, out hit, distance, collisionLayers))
         {
             newPosition = hit.point + (hit.normal * collisionOffset);
         }
     }
 
     /* Keeps the angles values within their specificed minimum and maximum
      * inputs while at the same time putting the values back to 0 if they
      * go outside of the 360 degree range */
     private float ClampAngle(float angle, float min, float max)
     {
         if (angle < -360)
             angle += 360;
 
         if (angle > 360)
             angle -= 360;
 
         return Mathf.Clamp(angle, min, max);
     }
 
     /* This is our custom logistic growth time signature with speed as input */
     private float TimeSignature(float speed)
     {
         return 1.0f / (1.0f + 80.0f * Mathf.Exp(-speed * 0.02f));
     }
 }

It looks like the script i grabbed already handles terrain collision... i just need to make sure to set the collision layers on it to include the terrain. But not sure how to do it.

What i tried:

I added now a new layer in the inspector called it Terrain. Then in the hierarchy on the Terrain i change it's layer to terrain. Also in the script i selected Terrain. But it's still not working.